Sub to complement KEF LS50s: SVS vs. Rythmik

Hi all. I'm considering adding a subwoofer to my current set up: Bluesound NODE2i (Tidal MQA) to Cronus Magnum II to KEF LS50s. I'd like to supplement the KEF's on the lower ranges.

I'm considering 2 subs and have located a few deals:

  1. Salk Sound Rythmik F12 in white for ~$1100 
  2. SVS SB4000 for ~$1200

I particularly like the room control on the SB4000, but am not wed to it. Any thoughts between the two? Anything else you'd recommend in this price range?

A few other notes: room is 12 x 30 feet, with a wall of windows (NYC condo). Acoustic paneling is next! Likely don't need all the size/wattage that either of these subs provide. Perhaps getting a MartinLogan "X" with the room correction could suffice, but would rather buy top notch and be done with it.

I'll utilize this for 75% music / 25% movies. Music tends to be bluesrock/jam bands/jazz

Paradigm X12.   I’d bet you can get a dealer down to around $1,1050 if you’re nice.   It’s an incredibly musical sub and has probably the best room correction available on any sub.

See if you can borrow one from a local dealer.  I guarantee you will be blown away.
Thanks. I'll definitely have a look!

     Through experience, I've discovered that one sub is able to provide good bass performance at a single designated listening seat if located properly utilizing the 'crawl' method but you may find it difficult, depending on the main speakers utilized, to configure it so that the bass seamlessly blends with the sound performance of your main speakers' reproducing the mid-bass or midrange on up to the treble. The bass may sound as if it's lagging behind and/or disconnected from your main speakers, especially on fast, smooth and detailed speakers such as electrostatic and planar-magnetic panels.  I know the KEF LS50s are very good bookshelf speakers but I'm not certain if they're fast, smooth and detailed enough to cause integrating seamlessly with a single sub an issue.
     I understand you'd prefer to buy a single top-notch sub and be done with it but, unfortunately, the truth is that attaining good in-room bass response is not that simple no matter the price or quality of the single sub.  The reason this is true really has more to do with the quantity of subs in a given room and how they're positioned, than the quality of the subs utilized. Better quality of subs never degrades from good bass performance, it's just not as important as most would assume.  I'll try to explain why.
      Two properly positioned and configured subs in a given room typically provides bass response at a designated listening seat that's approximately twice the quality level of utilizing a single sub. Two subs provide increased bass output capacity and impact as well as increased bass dynamics due to the sharing of total bass requirements between two subs operating well within their limits and stress free with ample power reserves for sudden bass output dynamic demands.
    Psychoacoustic principles also begin to be applied beginning with the use of two subs in any given room that results in the bass being perceived as smoother, more detailed and better integrated with the main speakers.  
    To understand how this psychoacoustic process works, it's important to understand how bass soundwaves behave in a room with a single sub. Soundwaves increase in length as the frequency decreases and deep bass tone soundwaves are very long. A full cycle soundwave of a 20 Hz deep bass tone is 56' long, a 30 Hz is 36', a 40 Hz is 28' and a 50 Hz is 23'.  For reference, a full cycle soundwave of a 20,000 Hz high treble tone is a fraction of an inch long.  This mainly explains why humans are easily able to determine the originating source location (localization) of the shorter soundwave and higher frequency tones above 100 Hz and are unable to do so on the longer soundwave and lower frequency tones below about 100 Hz.
     It's also important to know three facts:
1. Our brains can't even process the presence of a deep bass tone until the full cycle soundwave exists in the room and our ears have inputted this information into the brain.
2. Our brains require the input of at least three full cycle bass tone soundwaves before we're able to recognize a change in bass volume and pitch.
3. Our brains cannot localize deep bass tones (detect where the sounds are coming from) with frequencies below 100 Hz.
    With the deep bass soundwaves being longer than any room dimension in many individuals' rooms, this means the soundwave will leave the single sub and need to travel as far as it can in the room and then reflect off the first room boundary (floor, ceiling or wall) it meets then keep traveling in the reflected direction until it meets the next room boundary. This process continues until the soundwave runs out of energy and with each subsequent bass tone launched into the room by the single sub.
    These numerous bass soundwaves of various frequencies launched into the room by the single sub, and reflecting off room boundaries, inevitably run into each other at various angles causing what are called a Bass Room Mode at each specific room location at which they meet or collide. Depending on the specific angle at which the soundwaves meet, we perceive these bass room modes at specific spots in the room as either a bass overemphasis (bass peak), a bass attenuation (bass dip) or even a bass cancelation (bass null). The result is an overall perception of the bass from a single sub as uneven or 'lumpy'.
    However, when a second sub is properly deployed and positioned in the room, the very interesting and useful principles of psychoacoustics (how our brains process sound and our perceptions of it) begin to come into play, which results in a perception that the bass is smoother, more detailed, better integrated with the main speakers and more natural or realistic.
      Unexpectedly, this is accomplished through the second sub actually significantly increasing the number of bass room modes (bass peaks, dips and nulls) in the room. Our brains naturally and fortunately process the presence of multiple bass soundwaves below 100 Hz, by adding them together by frequency and averaging them out. This results in fewer bass modes being perceived in the room and a perception overall that the bass is smoother, more detailed, better blended with the main speakers and more natural.  
    Acoustical experts, such as Dr. Earl Geddes, Dr. Floyd Toole and others, have proven scientifically that in-room bass performance perception improves as more subs are added to virtually any given room, beginning with two subs and with improvements continuing up to the theoretical limits. Of course, there's a practical limit to the acceptable number of subs in a domestic room.
    I'm fairly certain the exact number of subs considered acceptable in a domestic room is higher for most men than most women but, interestingly, the scientists found significant bass performance perception gains were attained with each additional sub up to four but smaller more marginal gains were attained with each additional sub beyond four.

    Three properly positioned and configured subs in a given room typically provides bass response at a designated listening seat that's approaching the optimum quality level attainable at a single listening position. Three subs provide even further increased bass output capacity and impact as well as further increased bass dynamics due to the sharing of total bass requirements between three subs operating well within their limits and stress free with very large power reserves for sudden bass output dynamic demands.
     Psychoacoustic principles are more strongly applied with the use of three subs, as opposed to two subs, in any given room that results in the bass being perceived as even smoother, more detailed and better integrated with the main speakers.  
     My opinion is that the SVS SB-1000 (a small sealed sub with bass extension down to 24 Hz) and the PB-1000 (a slightly larger ported sub with bass extension down to 19 Hz) subs are ideal for utilizing in two and three sub bass systems because they're both very good quality, are relatively small, have all the necessary features/controls and are currently great bargains at about $500/each (slightly more for gloss black or white finishes). Here's a link to the SVS subs:

    The Salk Rhythmik, SVS SB--4000, a larger JL Audio or a Martin Logan sub all may be better subs with more features but, remember, the critical factor for in-room bass performance is the quantity of subs used in the room and the quality and features of each is much less important.  Besides, the reality is they're all high quality subs and room correction is not required for optimum performance on bass systems utilizing two or more subs.  
      However, if you prefer the best in-room bass performance,  the Audio Kinesis 4-sub Swarm distributed bass array (DBA) system is definitely the ultimate bass system that I'm aware of. This is a complete $2,800 kit that includes four 4 ohm unamplified subs that are each 1' x 1' x 28", weigh 44lbs and have a 10" aluminum long-throw driver and a 1,000 watt class AB amplifier/controller that powers all four subs and controls the volume, crossover frequency and phase of all as a group.  
      The use of a 4-sub DBA system will provide near state of the art bass performance not only at a single listening position but throughout the entire room. This is very useful if you have multiple seating positions in your room and prefer having very good audio at each position for both music and HT.
    Of course, only the designated listening position will be optimized for bass, midrange and treble response along with stereo imaging but very good full-range audio will still be provided at each seating position.
    They're also other benefits of the 4-sub Swarm DBA system.  Absolutely no bass room treatments are necessary. You'd just need to incorporate room treatments for the midrange and treble response on your main speakers (first reflection points on each side wall and possibly some treatment on the front and rear walls).  And these subs look very stylish in the room, kind of like hi-end wooden art gallery pedestals, since the 10" drivers face towards the nearest wall and have connections hidden on the bottom so all that's visible is three wood sides and the top in the wood of your choice.  My wife usually has a small vase with fresh flowers sitting on one of the two that are visible in my room.  They also make good end tables since they're an ideal height of 28 inches.  
    You also have the option of creating your own custom 4-sub DBA system, rather than using the complete Swarm system, by utilizing four SVS SB or PB-1000 subs for about $1,900 ( $950 discounted price for a each pair of SVS subs) or any four subs you'd like.  The only down side is that you'd need to configure the volume, crossover frequency and phase settings individually for each of any self-amplified subs used rather than once for all four subs as a group on the Swarm system. Here's a link to an Absolute Sound review of the A K Swarm system that is very accurate (I use this system in a 23'x16'x8' room with Magnepan main speakers and it works like a charm):

Best wishes,
Tim is correct!After reading about Tim's and other member's successful implementations of "the swarm",then doing further research, I added two more subs to the two SVS 1000 subs already in my system and am very pleased.Every frequency benefits when the bass is smoothed and well integrated with this method.Here's another link that may offer more than you ever wanted to know:)
Hello jtcf,

I’m really glad you had the curiosity to learn more about the 4-sub DBA concept, took the initiative to give it a try in your room and that it’s performing so well for you.
I always find it difficult to describe just how well the 4-sub DBA concept performs in virtually any room and with any main speakers. It’s a bit frustrating because I try to spread the word through lengthy and thorough posts that likely only convince a few people sufficiently to give it a try, such as yourself, while understanding all along that even a brief audition of a good quality 4-sub DBA system would probably convince literally everyone. I take some solace in knowing that I don’t think it’s possible to overstate just how exceptionally well this concept actually performs. I sense an almost magical quality to how universally well this concept works which I attribute to the amazing psychoacoustic dynamics involved, mainly how our brains process an abundance of bass room modes existing in any given room that causes our perceptions to be that the bass is fast, smooth, detailed and natural.
There’s also another Audiogon member, dbphd, I’ve been working with and he’s just awaiting two of his Velodyne 15" subs to be repaired before setting up his own custom 4-sub DBA system. He’s also be using KEF LS50s for his main speakers and some of his surrounds in his combo music and HT system. Jdtalv may want to follow any existing and near future threads from dbphd for some very relevant info.

Have you listened to the HSU VTF-3 MK5HP, or the VTF-15H MK2, subs they are both less than $1,000.00 and are very versatile in that you can use them for stereo installations (2 channel) or for HT set-ups.
What a great post. Noble 100, very informative. I am considering a sub, but reasoned 2 might be better. Now, maybe 3!! Or 4.... The science is fascinating. 
Yet another option is the Vandersteen 2Wq sub.  Very musical sub with a unique integration system.  Here's one available now...

I buy buy into the multiple sub thing too, but if you go with one sub you can do far worse than this Vandy for the money.  Best of luck. 
I have been using 2 SVS SB-1000s to give a bit more kick to a wonderful pair of PSB T2s for years.  My NAD 390DD DAC/amp allows for a flexible crossover point and low pass gain adjustment.  Adding Dirac Live to the mix helps even things out and lock timing down.

I would add that smaller subs are just less ridiculous looking in a room as well.  My SB-1000s (white finish with white grilles) both sit on the bottom shelf of some pretty standard maple bookcases.  No one even notices they are there.  The bigger subs you mentioned above are certainly capable - but there is no hiding something of that size.
But - if I had a more dedicated listening space where aesthetics were less of a thing - the Swarm system mentioned would be high on my list.  I have read about it several times over the years and am dying to give it a try.

     There's definitely an abundance of very good quality subs available, both new and used, that will produce good quality bass in any individual's room and system at a single designated listening position.  I suggest we all agree on this point and dispense with the typical forum phase of having a series of posts from various individuals that recommend the OP use the exact same brand and model sub that they purchased and use in their systems.
      If the goal is to provide useful information and advice to the OP to help resolve their audio concern or question, however, it's probably best we just skip ahead to the useful information and advice giving bits.  In this spirit, if the OP desires better than just good quality bass performance and integration with their main speakers at their listening seat, a very good solution is to utilize two of whatever subs they choose rather than a single sub.  
     The truth is that the number of properly positioned subs deployed in a given room and system is more important than the quality of the subs used in determining the overall bass performance quality perceived at the designated listening seat, even though utilizing higher quality subs will also improve the overall bass performance quality. 
      In general, two subs will perform twice as well as a single sub, four subs will perform twice as well as two and three subs will perform significantly better than two but not as well as four.  The ultimate bass system, in terms of bass detail, smoothness, impact, dynamics and overall quality, that I'm aware of is definitely the 4-sub DBA system, whether it's the complete 4-sub Swarm kit or a custom 4-sub DBA using even higher quality self-amplified subs selected by the user.

….."I suggest we all agree on this point and dispense with the typical forum phase of having a series of posts from various individuals that recommend the OP use the exact same brand and model sub that they purchased and use in their systems."....

But this is what the OP is asking for, which of the two subs he is interested in is the best, preferably from someone who has heard both.  It is pretty intuitive that the more subs you use the fuller your bass will be.  But as mentioned above, if your set up is in your family or living room having multiple subs may not be possible (although I have two).
You could try two Gallo Acoustics TR-3D subwoofers like I do with the LS50s (I actually have the older versions). Also live in a NYC apartment about the same size, and they fill the room for music and movies. They have a small footprint and aesthetics that don’t look bad with the speakers. I haven’t tried room correction just setting them to crossover at 70-50 hz. Have the two shooting from the front, but I think it sounds best with one in the front and one in the back. Hope this helps.
has anybody tried small.. like 8 inch or 6 inch subwoofers? Is their such a thing with proper extension and frequency response?
I’m currently using LS50 and two SVS SB-16 Ultra and I have been SUPER impressed with the SVS subs.  The only down side I can mention is size and weight.  Besides that, they integrate perfectly with the LS50 speakers.  The sound stage is wide and open with incredible 3D imaging.  Of course, most of the time with music the subs are invisible completely and barely moving.  I’m using 70hz crossover.  Most importantly, I recommend two subs versus one for all reasons mentioned above.  Good luck!
I think REL makes an 8 inch sub Ti/5?

Without trying to sound so intelligent, I think the SVS SB4000 is an excellent choice.  I've had a few subs in my systems.  I've had the Velodyne ULD 15, Velodyne F1500R, JBL 8400, and NHT's SubTwo.  Only the Velodyne's were as musical as these SVS's.  I go with a two channel application with NHT 2.9's driven by an ADCOM 555 MKII.  Again, these are the only subwoofers that have allowed me to say goodbye to my Velodyne's in my head.

P.S.  Because of all the glass in your house, I would recommend the isolation system by SVS to cut down some of the rattle & shake of the room.  It will just be tight and quick!

I use this sub with my ls50s.   8" active driver and 2 8" passive design.   matches well to the ls50s and delivers a lot of bang out of a smaller package than most.  Highly recommended especially if space is a consideration.

jetter:"But this is what the OP is asking for, which of the two subs he is interested in is the best, preferably from someone who has heard both. It is pretty intuitive that the more subs you use the fuller your bass will be. But as mentioned above, if your set up is in your family or living room having multiple subs may not be possible (although I have two)."

Hello jetter,
     Yes, the OP was asking opinions on two subs from those that have heard both.  He also stated he was just "looking for a top-notch single sub and just be done with it". 
     I decided to point out that it's not that simple and that a single sub in any given room will not deliver the level of bass performance some might expect but that two subs , properly positioned and configured, are capable of providing bass performance at a single dedicated listening seat that is roughly twice the quality level of what a single sub is capable of providing.

    It may be intuitive that the more subs you use the fuller the bass will be.  However, it's much less obvious and commonly understood that the more subs you use, the more detailed, faster, smoother, more natural and realistic the bass is perceived due to the effects of psychoacoustics. This is in addition to having increased bass output, impact and dynamics with each sub operating stress free and well within its limits.  I considered these facts as important for the OP and readers to be aware of when determining how to achieve better bass response in their systems and rooms.   I'm not sure what your point is, do you disagree with my view that these added psychoacoustic benefits derived from using multiple subs are important ?
     I do agree with you that accommodating multiple subs in a living or family room is a common and legitimate concern.  I had this same concern when I decided to purchase the 4-sub Audio Kinesis Swarm DBA system.  I use my 23' x 16' x 8' small to medium sized living room for both music and HT purposes.  I previously had a lot of experience using and accommodating fairly large single and dual subs in this room with minimal major room furniture rearrangement but also without significant bass performance improvements.
     I had done a lot of research on the 4-sub DBA concept and had very high expectations of significant bass performance improvements.  I think the prospect of finally attaining near state of the art bass performance in my room that was well integrated with my fast Magnepan 2.7QR speakers, that I had been  trying to achieve for about the previous decade, may have influenced my decision to commit to just positioning each of the four subs in my room where they performed optimally and dealing with any required rearrangement of room furnishings afterward as best as my wife and I could.  Luckily, I have an understanding wife who also enjoys high quality audio reproduction on music and HT.
      The Swarm subs are heavy at about 44 pounds/each but relatively small at 12" x 12" x 28"/each.  It also helps that the front firing 10" drivers are designed to be positioned facing and just a few inches away from the nearest wall, with all connections hidden on the bottom of each sub. 
     The optimum positioning of the subs in my room turned out to be two along the front 16' wall (with each about a foot away from the left and right corners) and one along each left and right 23' wall (with each about a foot away from each rear corner).  The only furniture rearrangement required was moving an 8' couch along the right 23' wall forward by 2 feet.  The front two subs aren't even visible from my listening seat near the rear 16' wall since they are each hidden almost directly behind each of my 6' x 2' panel speakers.  If you're having difficulty imagining my description, I have pictures posted on my profile page.
     My main point is that the Swarm's four relatively small subs blended within the room very well and better than my wife and I and probably most people would expect.  The bass performance in my system is now what I honestly consider state of the art on both music and HT.  The bass is whatever the content calls for, fast, quick and taut or deep, sudden, powerful and impactful that you feel as well as hear. 
      The bass is seamlessly integrated with the main speakers in their similar qualities of being very quick, agile, detailed and always smooth and natural but now with the added dimension of the bass being powerful, impactful, dynamic and fully extended down to a flat response on deep frequencies to 20 Hz.  
     My point is that I was more than willing, if it resulted in attaining near state of the art bass performance in my room and system for music and HT, to rearrange my living room furnishings to gain these benefits.  The fact that I only needed to move my couch a few feet was not only fortunate in my case, I think it's a lesson learned that may also apply in other individuals' rooms.  I believe that, ultimately, it's a personal decision as to what compromises individuals, along with their significant others,  are willing to make in their rooms for the sake of attaining near sota bass response performance.  I would just suggest that the relatively small and attractive subs utilized in the Swarm 4-sub DBA system are more likely to blend in as seamlessly visually into one's room as they do sonically with anyone's main speakers. l  think it may be more difficult if a custom 4-sub DBA is created using larger subs.

I own two 12" sealed Rythmik and super happy with them in an all audio setup. Looking at the frequency  response of LS50 it seems these start to drop right after 100Hz. Maybe a better option are Rythmik double 8" (F8??) subs that can play nicely up to 250Hz, even if you don't need to xo that high, it seems regular subs wouldn't work best with your monitors as they tend not to be optimal above 80 or 90Hz.
I own the LS50s and paired them with two REL T5 and it works well.  The LS50s do have a bit of a bass boost that is unfortunate.  Perhaps installing the plugs would help?  TBH I should have/didn't try but that might have flattened out the bottom.  Anyway I do like the REL particularly with the monoblocks because they get the same signal as the main speakers. 
Great thread! I’m new to this hobby, and this is my first audiogon post. I just connected a sub to my KEF Q150s this week. It was a game changer. But I find myself saying "game changer" every time I add a new component. I keep thinking I’ll hit the point of diminishing returns for my money, but that hasn’t happened yet.

My question to any/all of you is, how do you connect more than one sub to your system? I connected mine via the speaker level input--speaker cable from the integrated to the sub, then from the sub to the mains. Paul at PS Audio says the speaker level/high level input best because it preserves the sound characteristics of the power amp, which also allows the sub to blend better with the mains. My thinking is that 2-4 subs may require connecting via the line level input, no? If this is the case, would you say that the trade-off for using the line level is more than made up for by the quantity of subs? Also, should my second sub be the same brand/model as the first? Or does quantity of subs outweigh brand consistency too? Thanks
Hey Jdtalv,

I would say that it is a horse apiece.

I also was debating between a Rythmik F15HP and an SVS SB4000. My feeling was/is the Rythmik servo technology is a better design, I ultimately ended up buying two SVS SB4000 due to my interaction with both companies. Though my interaction was not bad with Rythmik, it was not as professional as my interaction with SVS and since Rythmik is direct sales only I was a little concerned about how I would be treated if I ever happened to run into a problem. They both have at minimum 30 day trial periods so you could try them out in your system.

I also agree that any high quality sub will more than likely work in your system as long as they have a low pass filter (usually incorrectly referred to as a crossover) and phase alignment capabilities. (not just a polarity switch)

Since you say this system is primarily for music you will want a SEALED sub, not a ported or a sub with a passive radiator, and you will want to plug the port of your KEFs. If the KEFs start to roll off at 100Hz you will probably experience directionality from the sub so I also would recommend a minimum of two subs.

Lastly you should also check out for some excellent integration techniques.
i had the ls50s successfully integrated with a single JL Audio D110 subwoofer and the sound was excellent.  
JL makes high quality sounding subwoofers and the tone of the bass is what they really do well, the tone of the notes is addictive.  slam and impact are there too.  
the LS50s with no port plugs had a natural steep rolloff at 70 Hz in my room.  point being that no filtering is required and there will be no directionality to a single sub.  
the JL D110 will be an excellent sounding solution that pairs very well with your amp too, just connect with the high level speaker connections.  the JL is superb at blending as well
"My question to any/all of you is, how do you connect more than one sub to your system? I connected mine via the speaker level input--speaker cable from the integrated to the sub, then from the sub to the mains. Paul at PS Audio says the speaker level/high level input best because it preserves the sound characteristics of the power amp, which also allows the sub to blend better with the mains. My thinking is that 2-4 subs may require connecting via the line level input, no? If this is the case, would you say that the trade-off for using the line level is more than made up for by the quantity of subs? Also, should my second sub be the same brand/model as the first? Or does quantity of subs outweigh brand consistency too? Thanks"

Hello classdstreamer,

     Welcome to Audiogon. I’ll answer all your questions but I first wanted to clarify something on your current hookup of a single sub with your pair of KEF LS50 mainspeakers. You didn’t mention which exact sub you are using but it seems like it has the capacity to be connected using either line level or speaker level connections. You stated: "I connected mine via the speaker level input--speaker cable from the integrated to the sub, then from the sub to the mains." This is fine as long as your sub is just reproducing the bass frequencies at or below whatever you have the crossover frequency on the sub set at and the sub has an internal filter that passes all frequencies above whatever you have the crossover frequency on the sub set at to your KEFs. Or, are you running the LS50s full-range?  
     In either case, can you post and name the exact brand and model number of your sub so I can verify a few things?  Here are the answers to your questions:

1. "Paul at PS Audio says the speaker level/high level input is the best because it preserves the sound characteristics of the power amp, which also allows the sub to blend better with the mains."  Is this true?

     Numerous individuals and some subwoofer companies claim this to be true. However, I’m unaware of any objective experiments, testing or even any objective explanations or information that verifies this as valid. The only supporting information supporting this theory are subjective claims from individuals and the marketing departments of subwoofer companies that, apparently, are unable to be verified objectively. 
     Based on my personal 40 plus years of experience setting up and using various subs in my systems, I have perceived no subjective audible benefit of connecting my subs via the speaker level inputs when compared to the line level inputs, and vice versa.
There is, however, a more logical and objectively verifiable benefit to using subwoofers that can result from the main speakers and the main amp(s) being relieved from reproducing the deeper bass frequencies and instead having the sub and its dedicated amp responsible for reproducing all bass frequencies at or below a specific frequency. In theory, the limiting of the main amp and speakers to reproducing only the midrange and treble frequencies(through the sub’s high pass filter if it has one), combined with the sub and its amp reproducing only the deeper bass frequencies, will result in improved performance in the entire audible frequency range of 20-20K Hz provided good blending or integration between the two frequency ranges can be achieved.
     I have personally and subjectively perceived these benefits with specific main amp, main speakers and sub system combinations. But I believe the significance of the perception of this benefit is dependent on the specific amp, main speakers and sub system components utilized.
     For example, my current system components are 1,200 watt class D monobloc amps, Magnepan main speakers and a 4-sub distributed bass array system driven by a 1K watt class AB amp. I perceived no overall sound quality benefits by limiting my main amps and speakers to reproducing only the midrange and treble frequencies. I actually prefer the overall sound of my amps and speakers running full-range and my four subs reproducing all deep bass frequencies in mono from 40 Hz and lower. This does cause an overlap and slight overemphasis on frequencies from 34-40 Hz but I think I must enjoy this. 
     My theory is that this was due to my amps being sufficiently powerful enough that the relief in not reproducing the deeper bass frequencies in my amps for my main speakers, that only have bass extension down to 34 Hz anyways, was not as beneficial as it would normally be on a combination of less powerful amps and speakers with deeper bass extension.
     I suggest you just compare the overall system sound with the speaker/high level connection to your sub with the line level connection and use the method that sounds best to you. I’ve learned the best way to blend or integrate the sound of one or more subs with the main speakers is precise adjustments of the sub’s volume, crossover frequency and phase controls. 
     The goal is to set the volume and crossover frequency as low as possible while still sounding good to you overall. Remember, best results for most individuals’ preference are achieved when they’re not constantly aware of the subs being engaged, it’s normally preferred that they only engage when the music or HT content calls for it.

2. "My thinking is that 2-4 subs may require connecting via the line level input, no?"

     Not necessarily, some subs are able to be daisy-chained together as more subs of I believe the same brand, and possibly the same model, are added. Daisy-chained just means sub#1 is connected normally and sub #2 is connected to sub#1 via a specified method, sub#3 is connected to sub#2 and sub#4 is connected to sub#3 via the same specified method. Usually daisy-chaining on most subs is possible whether the speaker level or line level connections are used for sub#1. But again, you need to state the exact brand and model number of your sub so I can verify your sub is capable of doing this.

3. "If this is the case, would you say that the trade-off for using the line level is more than made up for by the quantity of subs?"

     As stated previously, I cannot discern any significant difference between connecting a sub via the speaker/high level input and via the line level input but it’s possible that you can. However, I believe virtually everyone can clearly discern the bass response performance improvements in their systems/rooms resulting from increasing the number of subs utilized, beginning with adding a 2nd sub and continuing with each additional sub up to four. Any improvements gained beyond four subs will be much less significant and likely beyond the law of diminishing returns. 
     The improvements clearly noticed with each additional sub will include the expected progressive increases in the maximum bass output level and increased dynamics but also the less expected progressive increases in the perceived bass detail, smoothness and the level of blending or integration of the deeper bass from the subs with the mid-bass to treble from the main speakers.
     Overall system sound to expect will progress from two subs sounding twice as good as one sub, three subs being a significant improvement approaching state of the art bass response and four subs sounding twice as good as two subs and likely considered by a listener as state of the art bass performance for both music and HT content.

4. "Also, should my second sub be the same brand/model as the first? Or does quantity of subs outweigh brand consistency too?"

     It’s not required that additional subs are the same brand/model as the 1st sub but may be very useful and important if you plan on daisy-chaining the 2nd sub and plan on adding even more subs in the future.
Sub quantity does generally outweigh brand consistency but probably not if daisy-chaining of subs is desired in the future. It still may be possible to daisy-chain subs of different brands but it depends on the exact brand and model of your current sub and future subs.

     Please post the exact brand and model of the sub you’re currently using.

I'm using  REL T-5 i with excellent results. I stream from Qubuz using a Schitt multibit DAC through a refurbished Threshold 400A class A amp. The Kefs like lots of clean power. The REL presents a very clean representation of instruments like acoustic bass and piano. Connect the REL using your power amp's speaker terminals through the Speakon cable. 
Great info! So i have the LS50s in a small room (12x14) with a single SVS SB-1000 with an all solid state front in (Mytek + Bryston) except for my vinyl and feel like it integrates nicely.  One of the original pluses of SVS for me was their tuning service. You call in after purchase and they have a bevy of knowledgeable guys (typically subcontractors...mine was in Texas) who walked me thru balancing the system. I would love to try a swarm but with a small room like I have, there's just not the room, physically or aesthetically. 

My interest, for now, is whether or not a second sub would add anything to my listening experience....? System seems very revealing now and like many, I listen to jazz and other equally intimate source material.
I am just going to chime in, please take my advice for what it is, some random guy on the internet:

1)  Noble 100 is absolutely right, the more subs, up to 4 (more than 4 provide marginal improvements), the better off you are in terms of smoothing out response and improving sound quality.  There is no amount of DSP that can make up for doing this acoustically.  

2)  Most room treatments provide minimal help for frequencies less than 100hz.  To effectively work, they must be large, and to remove large peaks must have a very narrow bandwidth (look up a Helmholtz Resonator).  GIK has excellent room treatments for reasonable prices and will, if you provide your room information, make suggestions.  

3)  This a controversial point, but in my experience high passing your main speakers makes a big difference, as it removes their trying to reproduce bass frequencies that increase distortion of the bass driver in your main speakers.  For some reason, people will argue against this, but I don't see them arguing you should remove the cross-over between your woofer and tweeter?!!!

In the case of your KEF's, that driver goes well into the midrange.  I also agree with Richard Vandersteen, that to fully integrate a subwoofer, your mains should have solid extension one octave below the cutoff frequency.  Your KEFs are close enough that using a high pass with them at 80 hz will work.  

4)  The vast majority of what we perceive as "slow" or "bloated" bass are room nodes and slow room decay.  Unless so poorly designed (generally a vented enclosure tuned to get maximum output) that transient response (really group delay) is so high it becomes audible.  Virtually no well designed home subwoofers suffer from this.  (As a note, most people perceive vented subs as tubby, because they have greater output: until recently, many, even high end, sealed subwoofers had a roll off starting in the 40hz range, now fixed by DSP).  

My two cents, get two or three less expensive subwoofers, carefully place them, and cross over your mains.  Rythmik offers the excellent L12 (white for $619 if you want that color) or LV12F, SVS the SB-1000 or PB-100, HSU the VTF-2 or ULS 15, or the RSL Speedwoofer 10s for $399, which gets you 3 for $1200.  
mcreyn, a random guy who seems to have considerable knowledge coupled with wisdom.
Hello mcreyn,

     I agree with almost everything you posted.  I mainly agree with your 3rd point but would just add that any achieved overall system performance gains, made from high passing your main speakers, are dependent on the degree to which your main amp(s) and main speakers benefit by being relieved of deep bass duties. The higher the main amp's power and the higher the bass extension limit of the main speakers, the smaller I believe the overall system sound quality performance gains will be.

Hello dancub,

     Adding a 2nd properly positioned and configured SVS SB-1000 sub to your system, in my opinion, would result in the bass being perceived as sounding roughly twice as good as a single sub even in your relatively small room.  You'll notice a general sense of ease to the bass, an increase in maximum bass output and an increase in the realism of bass dynamics due to the total bass duties being shared between two subs   
 neither of which is operating near its limits and both having ample reserve power for reproducing the powerful bass dynamics of music heard live at a small venue.  
     You'll also notice the psychoacoustic benefits to the sound that result from the use of multiple subs in any given room, that come into play beginning with two subs producing bass in a room and that are further enhance perceived bass quality in degrees up to a practical limit of four subs.  The clearly noticed progressive bass quality improvements noticed with multiple subs in any given room are an increase in perceived bass detail, speed, smoothness and improved blending or integration between the bass reproduced by the sub(s) and the midrange to treble reproduced by the main speakers.  The overall sound will sound progressively more cohesive as subs are increased from one to four in degrees.
     I know the SVS SB-1000 subs are small in size but very good performers that are a steal at the current price of $499 each.  My opinion is that buying and trying a 2nd one is a low risk and bargain price to experience bass response performance that I'm confident you will consider approaching state of the art even in a smaller room such as yours. Worst case is you try it out for 30 days, don't notice the improvements and return it for a full refund. Best case is you try it out, discover you suddenly have exceptionally good bass response in your room/system and begin thinking of the further gains possible by adding a 3rd or the ultimate fourth sub.  Other important qualities of your sub is that it's relatively small, has all the important configuration controls and overall is an ideal sub to be used in multiple sub systems.
     I should also mention it's very important to precisely position and configure each sub once you enter the wonderful world of multiple subs. I have a lot of useful experience and knowledge positioning and configuring 1-4 sub bass systems gained through research and using them in my own system as well as setting the up for friends' and relatives' rooms/systems and I enjoy spreading the word on how well the concept works and assisting setting them up.
       Of course, the decision is yours but I'm also willing to help you out with the optimum positioning and configuration if you'd like (I'm retired and have the interest and time).  But I'm also sure SVS's call in support could also assist with this if you'd prefer.    
Wow, thank you for the input. Your last point is one I've wondered about over the two years I've had the SVS...positioning. I recently read...who knows where...that in spite of traditional wisdom (i.e. you can't tell where those frequencies are coming from), that location can make a serious difference. What I remember is that they should be side ways from the primary speaker and behind. My current one meets the first but not the second criteria...if I'm remembering correctly....but bass integration in my room is pretty reasonable.  Actually, what I have been paying attention to most recently has been the lack of crispness or depth of voices (particularly female singers). This may be source material but the ones I love (Holly Cole, Siri Svale, Patricia Barber) have typically been well regarded productions. I'm going to take a rain check on your kind offer to help with the bass things until after I try out your recent suggestions (above) for bass management and play around with positioning to see if that helps with this "crisp" issue.  In between my own over abundance of retirement hobbies.  Cheers and thanks!
Point 3. Right?

Thanks for the help. My system currently costs less than a pair of KEF LS50s.

KEF Q150s - I paid $300
Monoprice Monolith Stands - I paid $75/pair
Peachtree Decco65 - $400 craiglist special
Google Chromecast Audio - $35
Sub - $20 craiglist special

I initially thought the sub would degrade from the definition in the Q150s, but it enhanced everything. Having some sub was better than having none. The casing of the sub says it is a Jensen JS1000-A. But looking at the driver underneath, I see the driver was swapped out for a Kenwood--possibly a car driver. Sorry for any heart palpitations this may have caused, but this is the reality of budget Fi. You'll be happy to know that I could hear the difference between Spotify and 16/44.1 once I placed the Q150s on stands. And know I can't go back! (Spotify and 16/44.1 sounded identical with the mains on a desk.)

I currently run speaker cable from the Decco65 to the speaker level input on the sub, then speaker cable to the Q150s. But I bet the Decco65 pre-out doubles as a sub out. Have to double check, though. Still, the Decco65 only puts out 65w, so allowing the sub to take over the probably helps free up power for the mains as you described, noble100. 

I have the crossover on the Jensen set to 50hz since that is the 3db falloff on the Q150. And I have the volume set appropriately on the sub to achieve a flat sound signature. 

The only way I would it would appear that I could daisy chain subs with the Jensen is through the speaker level input. 

As far as a sub upgrade, I have considered both the Rel 5Ti and SVS 1000. Rel seems to be of the high level input persuasion.
jdtalv, the SVS, and I've never even seen one. 

  Your going to need all the signal processing you can muster to begin a worthy crossover between the two unless you've considered an even more sophisticated method of manual multi band parametric variable phase, gain, and Q DSP, in which case most any box will do.

  Read as many DSP setup procedures as possible for a better understanding of how using a higher crossover point and adjusting the Q can attain a smoother transition that will more closely match your main speakers presentation in the crossover region.   

  My experience with integrating powerful subwoofers with small speakers has been quick and easy using a sub that seems to be out of production because of the manufacturers success in another field of endeavor. This same manufacture patented servo monitoring and control and found a better solution three generations ago. (calvint, you gave up too soon)

  This is in no way a substitute for proper positioning or the use of multiple subs which are delightfully explained here, elsewhere. Have fun with it.  

I owned 6 different subs from that manufacture, starting with their original 80s design. My  Rythmiks represent a step forward.  

As I said, I've never even seen one.  

Nice to hear of your satisfaction though.
My 2 cents from the 4 years of listening and enjoying experience with this magnificent speakers.

Buy a pair of competent subs that can go clean and flat as high as 100-120 Hz, because LS50’s response starts to weaken at these frequencues especially if you have them far from the back walls. If possible, cross them over at 100-120 to further relax them and improve the mids. Keep the subs close to the ls50 and you will enjoy. 
You can leave them full range as well just dial the sub volume adequatelly.
I enjoy them with a single b&w asw608 sub and the bass is adequate at the listwning position and very clean and articulate.