Whatta Ya Think About Hsu Research Subwoofers?


I’m in the market to add one or possibly two subwoofers to my stereo set-up.  I would like to spend $1,000 or less per unit, and that puts some of the higher-rated units out of reach.  I came across an ad for Hsu, which I had not heard of before.  I didn’t want a Chinese product and it turns out that Hsu Research is based in California and founded by Dr. Hsu, who has a Ph.D from MIT.  Audio reviewer Steve Gutenberg gave one Hsu product a positive review.  I was wondering if any of you have experience with Hsu and could share your impressions/recommendations with me.

My existing set—up is:  Parasound P5 pre-amp with Parasound A21 amp;  Martin Logan 60XT tower loudspeakers.  Thanks!
bob540

Showing 18 responses by noble100

big_greg: " If "prodigious amounts of bass is your thing", what difference does it make how you get it, whether it’s speakers or complementing speakers with a subwoofer (or multiple subs)? Even if you just want a "realistic" amount of bass, again, why does it matter how you achieve that? A purpose driven device (the subwoofer) just might be better than one that’s trying to be a jack of all trades (the speaker)."

Hello big_greg,

     The main reason I utilize a 4-sub DBA system in my room is not to have prodigious amounts of bass but to attain the highest quality bass that is sufficiently smooth, fast and detailed to blend in seamlessly with my smooth, fast and detailed Magnepan 3.7i main speakers.
     My pair of 3.7i are 6’x2’x2" dipole 3-way panel speakers produce very high quality smooth, fast and detailed almost full range frequency performance all by themselves in my 23’x16’x8’ room. However, they only have a rated bass extension down to 35 Hz which most people, including myself, would accurately describe as deep bass limited.
     I believe the bass the 3.7is do produce is sufficiently smooth, fast, detailed and, of course, seamlessly integrated with the midrange and treble planar-magnetic drivers that many would not discern a need for external dynamic subs that are notoriously difficult to integrate with planar-magnetic and electrostatic main speakers.
     But I added the Audio Kinesis Debra 4-sub distributed bass array (DBA) system, which is almost identical to the AK Swarm DBA system reviewed on the link attached below, to extend the bass range of the 3.7is down to the audible limit of 20 Hz and improve the deep bass impact, dynamics and realism of music in my room. The AK Debra 4-sub DBA system very successfully achieved this for me while also integrating seamlessly with my main speakers.

https://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/

     You asked: " Even if you just want a "realistic" amount of bass, again, why does it matter how you achieve that? A purpose driven device (the subwoofer) just might be better than one that’s trying to be a jack of all trades (the speaker)."

     You answered your own question. Typical tower or panel speakers are trying to be jack of all trades. Subs and sub systems, however, are purpose built devices that have the dual benefits of being capable of reproducing bass down to its audible limit of 20 Hz and, more importantly, being capable of being independently positioned in the room to avoid the pitfalls of bass peaks, dips and nulls at the designated listening position in your room.
     Tower and panel speakers are typically positioned in a room and in relation to the designated listening seat to achieve the optimum midrange and treble performance along with maxinizing the imaging and the 3D sound stage illusion effect made possible by 2 channel stereo recordings and the playback through 2 precisely placed stereo speakers.
     This main speakers positioning is very important and enjoyable when done properly but the biggest loser in this normal process is bass performance at the listening seat. The precise positioning of the bass drivers (woofers) in the room and in relation to the listening seat is also very important for good bass performance. But they’ve been totally ignored during the normal process of optimizing the position of the main speakers for midrange, treble and imaging performance at the listening seat.
     Proper positioning of the woofers in the room and in relation to the listening seat has been totally neglected and their positions are usually relegated to positions directly below, and physically attached to in the same cabinet, the midrange and treble drivers. It is highly unlikely that the optimum position of the bass drivers in a room and in relation to the listening seat will be directly below the optimum position of the midrange and treble drivers in a room and in relation to the listening seat.
     What could possibly be the solution to this dilemma since the drivers are all permanently attached to each other in the same cabinet or panel and lack the capacity to be independently positioned for optimum performance?
You guessed it, the most basic solution is adding a single good quality sub that can be independently positioned in the room to supplement the bass already arriving at the listening seat from the main speakers and thereby optimize bass performance at the listening seat.
     Progressive improvements in bass performance quality at the listening seat will be achieved by adding good quality, properly positioned subs to the room. The very obvious and progressive bass quality improvements resulting from adding more properly positioned subs to a room are increases in bass smoothness, speed, detail, impact, dynamics, sense of ease, sound stage openness, size and naturalness along with a seamless quality to the integration to the main speakers.
     There are more details I’d like to describe about why it matters where bass performance quantity and quality improvements come from along with the benefits of multiple subs but I’ll stop here for the sake of brevity.

Tim
big_greg:" Tim, they were rhetorical questions. As I mentioned, I use 4 subs in my main system. My point was that there are a few people that make pronouncements that may be "true" for them, but they may not be truisms for everyone - "you have to have four subs", "subs are for home theater", etc. What's "best" for one system, one's listening tastes, and their room may not be for someone else."

Hello big_greg,

     Okay, but I just reread your prior post and you sure did a poor job of making it clear your questions were rhetorical and what your point was. Odd, because you were able to clearly make your point in one sentence,  "  My point was that there are a few people that make pronouncements that may be "true" for them, but they may not be truisms for everyone", in your last post quoted above but failed to clearly do so in your entire prior post.
     Good, that clears up the issue of your point from your prior post that wasn't initially clear to me and I agree with your recently understood point.  Thank you.  
     But I'm hoping you could clear up another section of your prior post that I didn't fully understand or had questions about.  You stated:
" The best subwoofer integration I’ve heard to date was done with two subs. Compared to that system, I feel like my 4 subwoofer system is more of a band-aid than a best of class solution. Not that mine sounds bad, it’s very good, but there’s always more than one way to skin a cat."

     I have 2 questions/comments:

1.  Can you elaborate on the this best subwoofer integration you've heard to date that only consisted of 2 subs?  It's not that I don't believe you, it's more a matter of curiosity.

2,  I generally agree with you that there's always more than one way to skin a cat.  Apparently just like you. I'm also very interested and familiar with the various methods available for this hobby.  For example, I typically utilize very different methods depending on whether the cat is dead or alive.  Is this what you were referring to? Can you elaborate? 

Thanks,
  Tim
bob540:
" I just posted a grainy photo in my profile showing the space I am working with.  Sorry it is so dark and fuzzy.  The doors to the right are to my garage and laundry room, so cannot block those.  The table that holds my electronics is centered under the TV — Martin Logan towers are to each side of that, but hard to make out.  There is space to the left, as I can move the tower that holds my CDs and the record albums.  I wish I had Bluetooth capability for the subwoofers, as I could place one further to the right out of the line of foot traffic, and then I have space for one on the left.  I could cover the cable with a small throw rug I suppose."

Hello Bob,

     Sorry, I got sidetracked having a bit of fun.
     I looked at the picture of your room and system.  It's clear enough that I understand your issues with space and traffic patterns.  I can't see the rest of the room, but I believe you mentioned earlier that there's a large sectional couch along the opposite 14' short wall in your 14'x19'x8' room.  Please correct me if I'm incorrect.
     The first solution that comes to my mind is to switch ends for your tv/equipment and your couch.  The back of your couch would be positioned about 3' away from the door to your laundry room/garage door to allow for traffic.  Your tv and equipment would be positioned along the 14' short wall where your couch was.  This format would allow for your main speakers to be moved out a bit from the wall with the pair of subs along this wall behind them.
     I understand there may be obstacles with this solution so I'll wait for your response before offering another possible solution.

Tim
Hello big_greg,

I definitely agree with you that it’s sometimes hard to pick up on rhetoric in posts since it just happened to me. I’m usually fairly good at picking up on satire, sarcasm and jokes but admit I may need some work on my rhetoric detection skills.
I think we resolved this little misunderstanding between us reasonably and amicably and really don’t think either one of us consider this a big deal or worth further attention. Thank you.
I noticed you wisely avoided answering my questions about our shared hobby of skinning cats. I was fairly sure you’d understand I was just having a bit of fun and have never skinned a live cat, at least as far as you know.
But thanks for elaborating on the very good 2-sub system your fellow music club member has.as well as your own systems. I really don’t consider myself someone who believes that what I use and like is what others should use and like. But I do realize I do get very enthusiastic about things that work very well in my system, such as the 4-sub DBA system, and I probably run the risk of being perceived that way on some of my posts.
I feel like I’m walking a really fine line with the 3-4 sub DBA system concept because it not only works so well for me even without any room treatments, EQ or DSP, it's also claimed to work well in virtually any room and with any pair of main speakers. This seems like the perfect exception to this axiom, a situation in which something that I use and like really is what others will likely use and like. It’s especially difficult not to get enthusiastic about a bass solution that apparently will work for everyone, regardless of their room and main speakers. Heck, not spreading the word seems to be the bigger crime with the 3-4 sub DBA concept.
As you and I know through personal experience, however, excellent bass performance can also be attained in a system and room by using as few as 1 or 2 subs. I think my best solution, when giving advice on this subject on future thread posts, is just to be honest and relate what I stated above on this post.
Thanks for spurring me to more fully explore and evolve my thinking on the subject of sub usage and effectiveness.

Tim
Hello Bob,

    Sorry I took so long in responding, Thurs-Friday were spent dealing with a water leakage in my main floor half bathroom. Everything's now not leaking and under control but I had to get my homeowner's insurer involved to repair the damage.
   Unfortunately for you, I think slaw has a good point: " If you get a sub/s, you have no good choices for optimal set-up." I like fireplaces and large windows in a home, but they can sure make things difficult for optimally positioning everything for good audio/video systems.
    But I still think 2 subs will perform and sound much better than one, we just need to figure out how best to incorporate and optimally position them in your room. What do we do when the going gets rough? No, we don't give up and quit. We keep searching for solutions, out of the box if necessary.
     Awhile back on this thread, bigwave1 suggested Syzygy subs that are reasonably priced, can operate wirelessly and have received several good reviews. Here's one of the review examples I found and read:

https://www.tonepublications.com/review/syzygy-slf-850-subwoofer/

     If you have a smart Apple or Android cell phone, this could be a very good solution for you, no cables to run plus you can use your phone as a remote and for taking advantage of their room correction capacity at your listening seat after optimally positioning both in your room utilizing the crawl method.  
     If you agree that a pair of the $800 Syzygy SFL-850 subs are a good solution for your room, I suggest you still start at the right front corner of your room, just to the left of the door to your laundry room and garage, and proceed in a counter-clockwise direction when using the crawl method to optimally position sub#1. From my experience, I expect this optimum position to be along your short front wall where your equipment rack is located, but I can't be certain, so your goal is just to position sub#1 where the bass sounds best to you (powerful, dynamic, detailed and natural) with the volume initially set at about 50% volume, the crossover frequency set at 40 Hz and the phase set at "0" (in-phase).
     For sub#2 positioning, just repeat the same procedure continuing around the room from where you positioned sub#1. I have no expectation where sub#2 will sound best in your room, so I'd just recommend taking your time, listen closely and trust your ears.
     I should let you know that I've never heard any Syzygy subs but from what I've read, I think they seem like a good option for you to try. If you're interested, I suggest you contact Syzygy and see if they offer a free in-home trial period for a pair to audition them. I really like these types of offers since they take almost all the risk out of buying them. Please let me know your thoughts.

Tim      
bdp24:" A Parasound amp will be fine with them, and as I said the subs came with a passive x/o, so no extra electronics are required. The two woofers can be replaced, or even just re-coned, which I was thinking of doing. I don't know what the price to have that done would be."

Hello bdp24,

     It seems like you're not aware that Bob is using his stereo Parasound A21 amp to power his ML 60XT main speakers.  He can't use this same amp to drive a pair of passive subs, he'd need another separate amp to power a pair of passive subs.  Or, he could use a pair of traditional self-amplified subs.

Tim  
Hello Bob,

     Absolutely nothing wrong with taking your time and getting the right subs for you.  Kind of like measuring twice and cutting once.
     I'm a bit surprised you weren't more interested in the Syzygy wireless subs that are about the same price for a pair and they seem ideal for your situation.  I'm just wondering why you seem to not want to give these a try?  Here's the review again:
https://www.tonepublications.com/review/syzygy-slf-850-subwoofer/


Tim
Hello Bob,

     Forgot to mention, driving yourself crazy with this audio stuff is a choice and one I don't recommend.  I consider this audio and music hobby a journey and try to enjoy it along the way.

Keep it fun,
    Tim
Hello Bob,

     This is a direct quote from you on your post from 2/24/20: " There is space to the left, as I can move the tower that holds my CDs and the record albums. I wish I had Bluetooth capability for the subwoofers, as I could place one further to the right out of the line of foot traffic, and then I have space for one on the left. I could cover the cable with a small throw rug I suppose.
     Bob, the Syzygy subs are exactly what you wished for; Bluetooth capable subs that are wireless and let you use an IOS or Android smart phone as a remote. You can place one to the left of your rack and the other anywhere in your room without running any wires. No throw rug to cover the cables is needed since there are no wires or cables connecting the subs except a power cord for each to the nearest a/c outlet..
     They’re made in the U.S.and there are 2 versions which have either 10 or 12" drivers:
The Syzygy SLF850 utilize 10" drivers, have a rated bass extension down to 24 Hz and are priced at $799 +$60 shipping=$860 each on Amazon.
The Syxygy SLF870 utilize 12" drivers, have a rated deep bass extension down to 20 Hz and are priced at $999 +$50 shipping=$1,050 each on Amazon.
Here’s a link to both on Amazon:
https://www.amazon.com/Syzygy-Acoustics-SLF-870-Subwoofer/dp/B01KBTWAL6?th=1

I     f you’re serious about your wish for Bluetooth capable subs and attaining very good bass performance in your system, I suggest you begin with a pair of 12" SLF870 subs, which will provide very good bass at your designated listening seat down to 20 Hz.
     Then in the future when you’re ready, you could add a third 10" SLF850 sub, which will provide even higher quality bass quality not only at your designated listening seat down to 20 Hz but throughout your entire room. Even though only 2 of the 3 subs are reproducing deep bass down to 20 Hz and 1 sub just to 24 Hz, the overall bass will sound like it extends to 20 Hz in the room.
     This results because adding the third sub creates the necessary minimal number of subs in any given room to qualify as a distributed bass array (DBA) system. The 3-4 sub DBA system concept has been scientifically proven to modify the bass sound wave behavior in any given room in a specific manner that creates a bass sound wave environment that our brains process and perceive as the overall bass being fast, smooth, extended, powerful, dynamic, detailed, natural and seamlessly integrated with the main speakers.
     I realize this all likely sounds a bit confusing to you. In general, the 2 sub solution works well at a single designated listening position while the 3-4 sub DBA concept solution works significantly better, as in about twice as well, throughout the entire room and like a charm. An Audio Kinesis Debra 4-sub DBA system has been providing near state of the art bass performance, for both music and HT in my room and system, for over 5 years now and I see no reason it wouldn’t do the same for music in your room and system with a custom 3-sub DBA system provided the subs are properly positioned and configured (configured meaning the volume, crossover frequency and phase controls are all properly set on each sub, which I can assist you with).
     However, I think you need to decide whether you want to utilize and run line level cables to traditional self-amplified subs like the HSU or utilize the newer technology Bluetooth wireless subs like the Syzygy.
     There’s also a third option available, which is to buy a Dayton SA1000 1K watt class AB sub amp/control unit for $300-400 from Parts Express (which the AK Swarm and Debra 4-sub DBA kit systems utilize) and use 2-4 less expensive good quality new or used passive subs connected via speaker wires. You’d also be able to progress with this amp from using 2 passive subs in your room to using 3 or even 4 subs. The main downside is running and concealing the speaker wires to them.
    All 3 solutions will produce equally very good results with a pair of subs and will produce near state of the art bass results with 3-4 subs. I’m willing to assist you with any option you choose.
     It seems like the wireless Syzygy subs may be a bit more expensive, mainly due to the shipping charges, but they’re also a lot more convenient, especially in your room. Are you willing and able to run speaker wire or line level rca cables to other areas of your room to save some dough? Or, do you prefer convenience? Let me know.

Tim
Hello Bob,

     You should know that I've never personally heard many of the subs discussed thus far on your thread, including any from HSU, ML, Syzygy and SVS.  I have, however, personally owned and listened to many brands and models of subs from JL, B&W, M&K, Vandersteen, Klipsch, Polk, Audio Kinesis and REL.  For years I experimented with these subs in various quantities and room positions in an attempt to attain the highest quality bass in my system and room, initially with single subs, then dual subs and ultimately my search ended very successfully just over 5 years ago when I tried the Audio Kinesis Debra 4-sub DBA system in my room.       
     I was pleasantly amazed at the immediate and huge improvement in the impact and quality of the bass in my room produced by this 4-sub DBA concept system as well as how seamlessly it integrated with my Magnepan 2.7QR main speakers that only had a rated deep bass extension of 35 Hz.  I've been using this system ever since with outstanding results on both stereo music and on my home theater 5.1 surround sound system.
     I've read statements from various acoustic experts that 3-sub DBA systems are capable of bass performance nearly as good as 4-sub DBAs if each of the 3 subs are properly positioned and configured.  I don't doubt this is true but I've never personally bothered to verify this in my room. But I've read numerous Audiogon posts from members stating that 3 sub DBAs have worked extremely well for them in their rooms. 
     My general advice, based on my experiences, is that 2 subs perform about twice as well as 1 sub and 4 subs perform about twice as well as 2 subs.  Based on what I've read, I really just assume that 3 subs will perform very well but perhaps not quite as reliably as well as 4 subs.
     However, I'm certain that a pair of subs, properly positioned and configured, are more than capable of providing very good bass performance at a single designated listening seat in virtually any room.  The benefits of adding subs beyond 2 in the room will typically be perceived as the bass having increased power, impact, dynamics and a sense of ease quality along with it being faster, smoother, more detailed and this high quality bass will be perceived throughout the entire room, not just at the designated listening seat.
      By now it's obvious that I tend to sacrifice brevity for the sake of detail on my posts, Sorry.  Finally, you asked:
" If I am considering Bluetooth subs, how about this one from Martin Logan.  12” drivers with Bluetooth capability, and they look similar to other models costing more (which doesn’t mean they sound as good . . Don’t know):

https://www.amazon.com/MartinLogan-Dynamo-12-inch-Wireless-Subwoofer/dp/B002O3W2SO/ref=pd_di_sccai_1... "

     Since discovering and installing the A K 4-sub DBA system, I'm no longer reading about, researching and listening to the numerous subs available.  I still haven't listen to the Martin Logan 1000W sub but I did research it a bit and read a few very positive reviews.  This sub has a rated bass extension down to 22 Hz, has the necessary separate controls for volume, cutoff frequency and continuously variable phase, has the correct L+R rca inputs to connect to your preamp, has an updated wireless system, costs less than the Syzygy subs and looks cooler.
     What's not like?  I think a pair of these to start seem like an excellent choice for your room, system and circumstances.

Tim
Hello Bob,

     I actually wrote and posted my last thread post before I even read  your post about your decision to buy a pair of SVS SB-2000 subs.  I believe you wrote your post before me but somehow it was delayed in being posted because I didn't see just read your post on Saturday night.  I don't want to take back or change anything I stated but, given your room, I am a bit surprised you decided to buy wired subs and not choose the more convenient option of wireless subs. 
     
     However, it sounds like you have some ideas and plans about how to run and conceal the wires you'll need to run from your preamp to a currently unknown exact location for the second SVS SB-2000 sub that needs to be located along one of the 3 remaining walls in your room ( a wall other than the front short wall where your tv, equipment rack and main speakers are currently located, as well as the wall the first SVS SB-2000 sub will be located.). 
     Can you state your plans about how you plan to connect both SVS subs, using line-level rca cables from your preamp or using high-level speaker wire connections to your amp? 
    And about how you plan to run the wires past the the 2 doors in the right front corner of your room?

Thanks, 
  Tim
Hello Bob,

     Okay, it seems like you have a good plan for routing the required  single rca cable from your P5 preamp's Sub Output 2, past both the doors to the line level input on the 2nd of your SVS subs.  The 1st SVS sub, positioned to the left of your equipment rack, will also require a single rca cable run from your preamp's Sub Output 1 to the SVS sub's line level input. 
     As I stated earlier, I have no doubt that you'll be able to attain good quality bass performance at a single listening position in your room utilizing 2 good quality subs like the two SVS subs you bought.
     But it's very important you understand that just plopping down each sub where it's convenient, you have the space or where you prefer them being located, is a highly unlikely path to success. Ideally, the best process is to use the crawl method to determine the exact position in your room for SVS sub#1 at which the bass sounds the best at your designated listening seat.  Then with sub#1 optimally positioned and playing, the crawl method is used again to determine the exact position in your room for SVS sub#2 at which the bass sounds the best at your designated listening seat.  The final step is to optimally set the volume, crossover frequency and phase control settings on each sub.
     By determining that you only have one possible position for SVS sub#1 in your room which is to the left of your equipment rack along your front wall, however, this ideal locating process has already been compromised.  It's possible you still might be able to attain good bass performance at your listening seat but I think this would definitely require you to use the crawl method to optimally position SVS sub#2 in your room.  The exact optimum position in your room to locate sub#2, however, will probably not be on the ledge that is part of your fireplace. 
     I suggest you buy the longest rca cable you can because the optimum exact location for sub#2 could be anywhere along the perimeter walls of your room; further down your fireplace wall, along your rear wall or even your wall opposite the fireplace.  Using the crawl method, it will be fairly obvious when you find the optimum room position, the bass will suddenly improve in its quality and become more solid, smooth, fast, detailed, realistic and natural.  
      In the meantime, it would be useful to reread your preamp manual's sections on about how to connect a pair of subs beginning on page 10 of the attached P5 maunual below:

https://parasound.com/pdfs/P5Manual.pdf  

Tim   
Hello Bob,

     The subwoofer crawl method is just a simple but very effective method to determine the optimum position in the room for each of your subs without using any expensive equipment, just you and your ears required. Here's a  a link to a video explaining how to do it:

https://www.youtube.com/watch?v=AV3oLLMgS-M

     If you'd like to learn more about utilizing dual subs in a room, along with the typically most successful room positions, I suggest you use Google because there's a lot of info on this subject.


Tim
Hello bob540,

     I've never owned a HSU sub but wouldn't hesitate to purchase them based on their very good reviews, reasonable pricing and generous return terms. 
     However, I would advise buying a pair rather than just a single sub.  Based on my experience, a pair of subs will perform and sound about twice as good as a single sub.  Bass is cumulative and having 2 will provide a more accurate, realistic and natural presentation of the bass impact and dynamics recorded on the source content with neither sub operating anywhere near its limit.  
     Having 2 subs running in mono mode, properly positioned in your room and in relation to your listening seat, will also provide the normal multiple subs in a room benefits of the bass being smoother, faster, more detailed and better integrated with the main speakers than a single sub is capable of.  The soundstage normally sounds wider, deeper  as well as more realistic, open and natural.
     Besides choosing a pair of HSU subs that match your physical size and budget requirements, you need to make sure the subs have separate controls on each for volume, cutoff frequency and continuously variable phase, I believe most if not all HSU sub models have these. If your budget is tight, I recommend that utilizing 2 smaller and less expensive subs will perform better than a single larger and more expensive sub in virtually any room and system by a wide margin.
      Useful skills to learn are how to optimally position and configure 2 subs in a room.  I suggest googling the 'crawl method' for positioning and setting the volume and cutoff frequency controls on each sub as low as possible with the bass still sounding good to you (powerful, dynamic, detailed and natural).  Remember, the goal is not to constantly hear and feel the contributions of the bass from the subs but for them only to  supplement the deep bass when the content calls for it.

Best wishes,
     Tim
Hello rocray,

     Yes, 3-4 subs properly positioned and configured in a room qualifies as what's called a distributed bass array (DBA) system that's been scientifically proven to be one of the most effective sub-based bass solutions that can also be utilized in any room and with any pair of main speakers, even those very fast and detailed speakers considered very difficult to integrate subs with such as planar-magnetic and electrostatic type speakers.
     In my experience, a3-4 sub DBA system will perform and sound about twice as good as utilizing a pair of subs. Adding 1-2 subs to a pair of subs and positioning them in a distributed bass array configuration results in a further improvement in bass performance in terms of speed, smoothness, detail, power, impact and dynamics.  It will also extend this near state of the art bass performance  throughout the entire room, not just at a single listening position as is the case with using a pair of subs. 
     I utilize a 4-sub DBA complete kit system, an Audio Kinesis Debra system, with results I find very accurately described in this Absolute Sound review of the Audio Kinesis Swarm 4-sub DBA complete kit system (which is identical in price and performance to the AK Debra system but the relatively small subs are more rectangular than the Swarm's squarer shaped subs.):

https://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/

     My advise is that multiple subs will provide significantly better bass response than a single sub is capable of providing, no matter its size, quality or price.  If your goal is to optimize the bass performance in your system, or prefer near state of the art bass performance throughout your entire room rather than just at your designated listening position, then I recommend using either an AK complete kit 4-sub DBA system or a custom 3-4 sub DBA  system utilizing your choice of subs.
     If you want to learn more about the effectiveness of the DBA concept, 3-4 sub distributed bass arrays and in-room bass response in general, you can google " distributed bass array concept" or "distributed bass array system".

Best wishes,
     Tim

     I read that contuzzi and corelli wisely use a pair of subs in their systems.  But I'm just curious if other posters on this thread have experienced the many very significant bass performance quality improvements realized when multiple properly positioned and configured subs are deployed in their rooms.  
     If anyone is still utilizing just a single sub in their room/system, I can assure you that adding a 2nd sub will provide an obvious and highly significant improvement in bass performance quality. 
     Utilizing 3 or more subs will provide near state of the art bass performance quality that I'm fairly certain will stun and amaze you, as first experiencing  the bass performance quality of a 4-sub DBA system in my room and system stunned and amazed me about 4 years ago.  I honestly don't believe it's possible for me to overstate how effective the DBA concept actually works in virtually any room and with any pair of main speakers.
     The only precautions worth mentioning are that you'll need separate controls on each sub for volume, crossover frequency and continuously variable phase if you create your own custom 3-4 sub DBA system using traditional self-amplified subs of your own choice and you'll need to set these controls individually on each sub.  If an Audio Kinesis complete 4-sub Swarm or Debra DBA system is used, however, all 3 of these controls exist on the supplied Dayton SA-1000 class AB 1K watt amp/control unit and only need to be set for all 4 passive  subs this amp powers in mono mode.  Each sub is 12"wx 14.5"dx 28"h, weighs 44lbs, contains a 10" aluminum long-throw woofer and is rated at 4 ohms.
      Another option for those who are handy and so inclined, is to buy a Dayton SA1000 sub amp/control unit for $300-400 from Parts Express and build 3-4 passive subs of your choice as to size, design and driver.
This option also has the benefit of only needing to set the volume, crossover frequency and phase once for all passive subs this amp/control unit powers in mono mode.  All sub wiring connections are made in series/parallel.  Depending on the choices made on the DIY subs, however, the total cost could exceed the $3K price of either of the AK complete 4-sub DBA kits.

Tim
Hello bob540,

     Each of your ML 60XT speakers have rated bass output from their dual 8" woofers of 35 Hz +/- 3 dBs. Humans are capable of hearing bass frequencies down to 20 Hz. Virtually all commercially available recorded music (CDs, LPs and digital music files) have all of the left and right channel bass picked up by the recording mics, that are below about 100 Hz, summed to mono during the recording’s mixing process. This is done because humans generally cannot localize (determine exactly where bass sound tones are coming from) on bass frequencies below about 80 Hz.
     In practical terms, the above means there’s no such thing as true stereo deep bass because we can’t determine where bass frequency sounds below 80 Hz are coming from and, even if we could, virtually all of the recorded music we play on our systems is summed to mono below 100 Hz anyways. So, it makes no difference whether we setup our playback systems for true stereo deep bass or mono deep bass reproduction since we’re going to perceive deep bass as mono either way. It’s also important to know that we get progressively better at determining exactly where sounds are coming from as the sound frequency gets higher, from about 80 Hz all the way to our audible high frequency sound limit of about 20,000 Hz.
     But it gets more complicated than this and all hope is not lost due to our remarkable brains and the fact that that all deep bass fundamental tones have overtones or harmonics that are at frequencies that reach frequencies well above 100 Hz, are recorded in true stereo, are played back through our main L+R main speakers and that we are able to localize (determine exactly where these bass overtones/harmonics are coming from).
Fortunately, our brains are able to associate these bass overtones/harmonics that are at frequencies that reach well above 100 Hz, we are able to localize, that are played through the main stereo speakers, with the deep bass fundamental frequency tones that are below 80 Hz, that we are not able to localize, that are played through the subs and, therefore due to our brains sound processing power, we are effectively able to determine specifically where deep bass tones below 80 Hz are coming from.
     This cerebral association process is very relevant to audio because it allows us to perceive exactly where in the stereo sound stage illusion that deep fundamental bass frequency tones are coming from. For example, the upright bass can be properly perceived as being located at the front left of the sound stage and the deep bass drums can be properly perceived as being located at the center rear of the sound stage. I’m sorry for all the detail but I think this information is essential in understanding how to attain improvements in bass performance quality in our domestic sized rooms and systems.
     As I mentioned at the beginning of this post, your ML 60XT main speakers are rated to have bass extension down to 35 Hz, which means you’re currently not hearing or feeling any of the deep bass content on any of the music you play from 20 Hz to about 35 Hz.
     This is basically the entire bottom 2 octaves of the musical spectrum, that forms the solid foundation of most musical genres and that I’m not even sure you realize has been completely absent from every track you have ever played or will play on your current system.  But I believe your interest in adding a pair of subs is likely based on your sense that a large and critical portion of the audible frequency spectrum has been missing in action from your system and musical enjoyment. So we’re in absolute agreement that your system needs a pair of good quality subs like Trump needs a brain and a soul.
     My opinion that a pair of HSU uls-15 MK2 subs you linked to are a very good choice. They have high quality deep bass extension down to the audible limit of 20 Hz, have the 3 required controls (volume, crossover frequency and continuously variable phase.) and each has the proper line level rca ’Sub In’ input for connection to your P5’s line level rca ’Sub 1’ and ’Sub 2’ outputs, respectively.
     I also think your P5 is an ideal preamp to use when adding a pair of subs to one’s system since it has dual line level rca mono sub outputs with a built-in adjustable low pass cutoff frequency that controls the range of bass frequencies sent to both connected subs. The P5 also has line level rca L+R main speaker outputs with a built-in adjustable high pass cutoff frequency that controls the range of bass frequencies sent to your
A21 amp first and then on to your main speakers. These are both very useful controls that we can discuss how to best utilize and set on future posts.
     Just one more opinion, I think you might want to consider paying the extra $150 per sub for the rosewood finish. You’re going to be seeing these subs in your room on a daily basis for probably at least a few years. I think the fact that you didn’t pay the relatively small extra money to have them better match your ML main speakers may nag at you long after you’ll remember or care about saving a bit of money in the short term. Just something to think about since it’s your money and decision.
     The last thing I want to mention is you’ll need 2 single mono male-to-male line level cords to connect each sub to your P5 preamp. If you use the crawl method to optimally position each sub in your room in relation to your designated listening seat, and begin your search at the right front corner of your room and proceed counter-clockwise around the perimeter of your room as recommended, I strongly suspect you’ll discover that the bass sounds best at your listening seat with both subs positioned along your front short 14’ wall. It could be with one in each corner but my experience makes me think they’re most likely to sound best with both subs along the front wall with one about 1-2’ away from the right front corner and the other 1-2’ away from the left front corner.
     I still suggest you just faithfully follow the crawl method and position them precisely where they sound best to you. But this means it’s best to buy one 8-12’ mono rca cable for Sub#1 and the one for Sub#2 much longer, at least to start with. If I’m correct about sub positioning, you could probably just exchange the extra long cable for another 8-12 footer or just buy another. But if I’m wrong, you’ll probably need it because the optimum position may be along your left long wall with the windows and you’ll need to avoid positioning Sub#2 directly over an hvac vent.

Later,
Tim
audioguy85:"Otherwise, keep the sub where it belongs, on a home theater set up. Maybe it’s just me, I don’t get it. Only an opinion, be nice..."

Hello audioguy85,
     Well, you're right about one thing, you really don't get it. 
     I think snapsc gets it and it's probably because he didn't let old and tired misconceptions and biases against using subs in one's music audio system stand in his way.  None of those things mattered to him or me.
     What matters to us is keeping an open mind about improving our systems and getting closer to the music.
     You state you don't get it and I believe you.  I've come to the conclusion that nobody, with myself and a surprisingly small group of others included, really gets or truly understands how amazingly well multiple subs in a distributed bass array actually perform until they  experience it. 
     I now believe it's simply that there are 2 groups of individuals: those who have experienced a 3-4 sub DBA system and those who have not.  The first group have all bought or created their own custom 3-4 sub DBA system and enjoy near sota bass performance on a daily basis for music, HT or both.  The second group, having never experienced a 3-4 sub DBA system, have no idea what they've been missing and remain blissfully ignorant. 
     So ultimately you're correct, you don't get it and you just don't get to enjoy near sota bass performance on your system and music on a daily basis.  Congratulations, but at least you still have blissful ignorance going for you.  

I was nice, right?
     Tim