Distributed Bass Array configuration


Please, I don't want to debate the merits of the DBA nor of those who espouse it. I am considering adding two more subwoofers to a system that has two already. To those who use a DBA,I am interested in how you have configured them, specifically--
  1. Do you run in mono, or do you split the array to run in stereo?
  2. What is your approach to setting phase (delay) among speakers that may be facing different directions and are different distances from the listener?
Thanks!
mike_in_nc
1. Doesn't matter. It is all mono. Proved it by running both ways. No difference. There is a lot of BS around subs. Stereo bass, for example.

2. This one is a misunderstanding of phase. The phase of the subs in relation to the mains doesn't matter. Timing does not matter. The importance of phase is in that the output of all the subs with the room, it all interacts. Each sub has its own set of room modes. These room modes move around when the sub is moved around- or when the phase of the sub is moved around. 

If you do your sub setup, place the subs asymmetrically at different distances from the corners, set crossover 80 or lower, and then set level, you are basically done. These are the big ones. Phase, I have it on my Dayton amps, as well as on my Talon Roc, and it is easily the lest significant of all the things you can do. 

All my subs by the way fire into the wall from about 2" away. They are asymmetrical, and I never have any idea where they are or indeed if they even are on at all! No one does! Yet the bass is beautifully 3D and seamless with everything else.
@millercarbon - Thanks for your comments.

Regarding sub phase, I have experienced its making a big difference in FR (measured and audible) between sub and main, but it’s easy to believe that with 4 mono subs not all set to the same phase, this would lose its importance.

I never thought of firing the subs into the wall. Interesting idea!

I do agree that with proper setup, subs can and should be undetectable, and I’ve accomplished that with 2 subs. Should be easier with 4.

I can imagine that using mono subs, one loses the need for symmetry, thus it should be easier to get smooth response.

Thanks again.
Hey Millercarbon, just copy that and paste that from now on.  Most straightforward, no bs explanation I’ve heard to date.

And for the record, I still hate subwoofers as much as I love them

I run my front subs in stereo because I’m using a very high crossover point. My rear subs are technically mono but I am running both the left and right channels to each sub. Phase on all subs is set to zero. I have a very small room so phase is not a big issue for me. I do delay the signal to the mains by a few milliseconds to compensate for the delay from the subs due to the e.q. circuitry built into each sub’s amp.


REW measurements definitely show a difference. I guess it's up to each person if it matters or not.
Here's some example of in-phase vs out-phase graphs. Scroll down in the link to view all 5 screen captures.

First graph is comparing dual subs in and out of phase.

Next 2 down include the phase graph for dual subs in and out of phase.

Last 2 dual subs for the Group Delay for in and out of phase.

https://imgur.com/a/RSQiBMi

From REW author.

The vertical lines for the phase graphs are called Wrapped Phase:

" Phase is cyclic, due to the periodicity of the sine function. When it is increasing and reaches 180 degrees it wraps around to -180 and continues with -179, -178 etc. When it is decreasing and reaches -180 degrees it wraps around to 180. The vertical lines show those wrap points. If you look in the graph controls you will see a button to "unwrap" the phase, which allows it to go to 181, 182 etc rather than wrapping, but you can end up with extremely large value ranges in unwrapped phase, partly because any time delays create a frequency-dependent phase shift that gets ever larger as frequency increases.

All that changes at the wrap points is the way the graph is drawn. Think of it more like reading the second hand of a clock, after 59 secs it starts again at 0 and counts up again, but time has not jumped or done anything other than progress as usual. The part of phase that gets plotted when wrapping is used is like showing only the seconds of a time."

Group Delay
" The Group Delay at a frequency is the slope of the phase at that frequency. It provides an indication of how the envelope of signals at that frequency is delayed in the course of passing through the audio chain - if everything is delayed by the same amount the shape of the signal is not changed, it simply arrives later, but delays that vary change the shape of the signal.

Anything that affects the phase will also affect the group delay, which includes crossovers, the response of drivers, the low and high frequency roll-offs of the various parts of the audio chain, any time delays in the system and the filtering effect of the room, its modal response and the frequency-dependent absorptions of its surfaces and furnishings".

OP, I have this inquiry happening now, too. Here is a very informative discussion that moves past some top level opinions, one way or the other. 

https://youtu.be/1X_n6kG9FOc

"We take a look at stereo vs mono bass in a multi-sub 2CH and home theater environment. Does Stereo bass have merit and is it worth going through the potential complications of configuring your system to accommodate stereo bass? Watch our Youtube discussion to find out."

Thanks b_limo it is very clear, yet evidently still not clear enough on one point: there is no such thing as stereo subs! Because, there is no such thing as stereo bass, at least not below 80.

This will be like, I don’t know how many times clarifying I am losing count. And apologies in advance for shouting. But when everything is done right it SOUNDS AS IF the bass is stereo, BUT IT IS NOT!

If I have one weakness it is not bothering to keep track and being able to reference every tiny little thing. So times like this I forget where it comes from. But some big engineer paid a lot of money did an in-depth study of a wide range of recordings and found none of them had stereo bass, and anyone interested can look it up.

Another good one to look up is my mega DBA review. Mine uses two Dayton amps partly because at the time I wasn’t sure and thought two might be better. Just in case. Once I had them though some of the first things I did was to try running my new DBA in all possible configurations. I went through the gamut. 4, 8, 16 ohms. One amp mono. One amp stereo. Two amps mono. Two amps stereo. Played with phase. Tried all the time waster put the sub in a chair while you crawl around on the floor. About the only thing I did not try is the even worse stuff that requires a microphone. When the day comes a mic is better than my ears, take me out and put a bullet in the head, I’m already dead.

BUT, and this is a very big important BUT, it sounds as if it is. Why? The answer can only be that information above the low bass is somehow integrated into one seamless whole. This happens in the brain. Not in the ears, but between them. For proof look no further than the Supertweeter thread where everyone who tries them says two things: 1. they can’t hear them- too high frequency- and 2. they improve the bass. So how does ultrasonic 60kHz improve bass way down below 60Hz? You tell me. But it does.

Here’s a key moment from the video above -- https://youtu.be/1X_n6kG9FOc?t=970

Point: there is a difference, because there is such a thing as stereo sub effect, but these are limited cases and depend on the recordings; plus, even when it is in the recording, it is not always easy to hear; plus, and this seems key, it may not be as effective as running all subs in mixed mono because that’s more effective in dealing with room modes. That’s what Poes says, at least.
Post removed 
Like always it depends. It depends on the crossover point which depends on the type of main speakers you are using. Any full range loudspeaker benefits from a higher crossover point. Any system with a crossover point above 80 should be using stereo subwoofers in a symmetrical array with the main speakers. Any line source main speaker set up should have a line source subwoofer array. This usually requires more than 2 subwoofers, 4 in most residential settings. If the crossover point is below 80 hz there is no problem I can think of with mono subs only benefits. It will not hamper envelopment at all. Having said this I prefer higher crossover points. I think if phase and delays are handled properly there are no disadvantages only benefits in headroom and distortion levels. This is crucial for full range main speakers, just my humble opinion. 
Another novice arguing that it can matter and it's not all mono. SOME benefits of running subs as STEREO. Paul McGowan. 
https://youtu.be/Pp-0vvqERAs?t=254
But what would he know?
Brian Ding includes a continuously-variable 0-180 degree (0-16ms delay) Phase Control on his top-level Rythmik subs for a reason. If a loudspeaker is mated with a subwoofer or two---the speaker and sub(s) used with an electronic cross-over set to, say, 80Hz, the loudspeakers receiving a high pass-filtered signal, the sub(s) a low pass-one---if the loudspeaker and sub are reproducing an 80Hz signal in opposing polarities---creating a null at that frequency, the two will produce a combined frequency response with a deep notch centered at 80Hz. The Phase Control is provided to make possible getting the loudspeaker and sub in phase at 80Hz, thus avoiding the deep hole in frequency response centered at that frequency. This is loudspeaker design theory 101.

As for stereo bass in recordings: some music contains two---or even three---different basses being played simultaneously. Brian Wilson has an electric 4-string, an acoustic (upright) 4-string (both producing a bottom note---E---41 Hz in frequency), and a baritone 6-string all playing at the same time on some of The Beach Boys recordings. But Brian mixed to mono, one reason being he wanted to determine the balance between all instruments and voices, not leave it to the mercy of the hi-fi speakers and their placement.

But some recordings made in large churches, cathedrals, and music halls (Classical, mostly) are well-known to contain out-of phase low-frequency information between the two channels in a true stereo recording, caused by the long wavelengths created in large rooms. Left and right channel mics may received sound from the same sources at different times---the very definition of phase. The bottom note played on an organ fitted with a 32 ft. pipe produces a 16Hz tone! Recording engineers work hard to keep the sound of the room intact, which requires maintaining that out-of-phase information at low frequencies (you can read about it---it’s in the literature.). If your music collection contains such recordings, please respect the engineer’s work, and use your front subs in stereo pairs. Additional subs may be in mono.

     Millercarbon has it right:

1.  There is no such thing as stereo bass below 80 Hz since it's a fact that humans cannot determine the originating location of any sound deeper than approximately 80 Hz.  This is the reason I also configure my Audio Kinesis Debra 4-sub distributed bass array in mono mode.  If anyone thinks they're able to distinguish the specific originating location of bass below 80 Hz, I'd suggest testing this for themselves with test tones.  These test tones are available on-line via a Google search.

2.  I also found the Phase setting is not very important in attaining excellent bass performance in a 4-sub DBA system.  Timing and arrival time of deep bass sound waves at an individuals ears are, therefore, also not anything to be overly concerned with since a 20 Hz bass tone sound wave is about 56 feet long, which likely exceeds the dimensions of at least the length or width of most domestic rooms.  See the linked sound wave lengths in inches as a reference below:

https://www.jdbsound.com/art/frequency%20wave%20length%20chart%202013.pdf

     Bass sound waves under 80 Hz also emanate outward from the sub driver and are dispersed into the room in an omnidirectional direction, which means these multiple sound waves have likely bounced off at least one room boundary (floor, ceiling and wall) before they reach an individual's ears at the listening position and they perceive a bass tone.  
     The truth is our brains are capable of summing and averaging these bass sound waves by frequency/tone, that arrive within about 10 milliseconds of each other, but we're still not able to determine specifically where these bass sound originated from if they're below about 80 Hz.
     Finally, even if our brain could locate where these multiple and bouncing sound waves/tones were coming from, there are apparently no examples of commercially available prerecorded music, in any format, that bother to record discrete left and right bass under about 80 Hz.  Recording engineers have been routinely summing left and right bass under about 80 Hz as mono, and sometimes even higher, as a standard practice for over 4 decades. 
     Why?  Because these recording engineers have known the truth about the myth of stereo deep bass for about the last 100 years, ever since scientists first discovered and formally established this fact through the scientific method.  Don't believe me?  Try and find any recording, in any format and any date, that has deep bass below 80 Hz recorded in stereo. 
     Perhaps some near future recording engineers will utilize the full capabilities of the latest and greatest hi-resolution direct to digital recording method and introduce the first commercially available music with stereo deep bass.  But I wouldn't hold your breath.

Later,
 Tim 
 
     
My Mezzo Utopia's go down to 35Hz.

Sometimes I turn my sub off the Mezzo's are so good at bass.


However, is the consensus to set a single Subwoofer's(Sub Utopia)crossover to 80Hz?
Hello vinylshadow,

      I believe most set their cutoff frequency according to their main speakers bass extension capabilities and personal preference discovered via trial and error experimentation. 
      My opinion is that 80 Hz is too high since that's the generally accepted borderline between bass being directional or nondirectional.  You can't tell where the bass is coming from below about 80 Hz but can begin to tell above this frequency.  
     The goal is to integrate the bass from 1 or more subs as seamlessly as possible with one's main speakers.  The subs should remain completely silent until the source recording contains bass content at or below the low pass crossover frequency setting.  The first step, which you've already done, is to find out the rated bass extension of your main speakers, which you stated is 35 Hz.
     By coincidence, my main speakers (Magnepan 3.7i)also have a rated bass extension of 35 Hz but I utilize an Audio Kinesis Debra 4-sub distributed bass array (DBA) system to supplement the bass performance.  My point is that your optimum crossover frequency is probably going to be a bit different than my setting (currently set at between 40 and 50 Hz) for seamless integration.  But I'll describe the procedure I used in the hope that it'll work similarly as well for your single sub as it has for my 4 subs:

1.  Optimally locate your sub in the room using the "sub crawl" method (you can google it).
2.  Play some music with good and repetitive bass content.
3.  Sit at your listening position, set the crossover frequency control on your sub to 60 Hz, set the phase control to"0".  Then have an assistant slowly turn up the volume control on the sub until the volume of the bass from the sub matches and blends with the volume of your main speakers as closely as possible.
4.  Still seated at your LP, have an assistant slowly lower the sub's crossover frequency as low as possible with the bass still sounding very good to you ( "very good" meaning the bass remains solid, powerful, detailed, dynamic, seamlessly blended with the main speakers and natural sounding). Remember, the goal is to not even hear the bass from the sub until the source material calls for it. 
5.  Still seated at your LP, have an assistant slowly increase the sub's phase control upward from the "0" position to the "180" position until the bass sounds best to you ("best" meaning loudest, most detailed. best blended with the main speakers and most natural).  Note that these changes in phase settings will be subtle and you may need to closely listen through several slow "0" to "180" cycles to discover the optimum setting.  Just do your best since this setting isn't usually critical.

Hope this helped,
      Tim 
@mike_in_nc  - 

Firstly, heed the advice from MC/Noble101/and especially Duke LeJeune.

Secondly, I will divert from phase, crossover frequencies, gain, etc. and instead share my experience regarding subwoofer placement.

I am running four subwoofers (yes, the AudioKinesis Swarm).  Two are mounted up high facing the ceiling.  Their heights vary with one 12 '' and the other 18 '' below the ceiling.

Duke mentioned in an earlier thread that any subwoofers mounted above the room mid height contribute to the vertical plane, thus further reducing standing bass nodes.

My A/B testing (4 on the floor vs. 2 on the floor with 2 up high) in 2 rooms (10 x 12 minus closets and 12 x 16 living room) at my previous house along with over a year at my new house (also A/B testing between a 13 x 16 vs. a 15 x 19 room) all clearly confirm a noticeable all around improvement.

Hope this is helpful to you!
I do like the fact SVS has excellent new subs with great apps 
and even programs you can use with USB Mike and online free programs and setting ,SVS 3000, and Ultra 16 are the only subs I know of with 50 + bitAnalog devices microprocessors and their Dual voice coil on these 2 models is special for when at low volume ,you use the smaller inside of the voice coil to have full control ,and when louder music is player the inner and outer voice coil is utilized .
my brother has 4-Ultra 16s in his Audio ,Home theatre setup .
the sound pressure is so immense when a low Bass note hits you can literally feel it . I have 2-3000s  , buy sealed not ported for best 
tightest Audio . , I will say SVS  tech support is the best I have encountered and I have had the top 4 brands 
and who else gives a 5 year transferrable warranty ,and free shipping ,and 45 day Audition  with even free return shipping !
no one but SVS , Even their latest mini 3000 , and $499 has an app. And I  had a JL Audio 13  before,Rel the SVS is just has much more technologies built in and theApp for phone or tablet is excellent to adjust Bass, or volume on the fly per recording if need be is great👍
Hilde45, very poor choice of experts. Mr McGowan is a businessman with poor hearing. His objective is to make money by any means possible. He is reasonable good at that. I suck at it. 
Most of these people making comments here on this subject are using 3rd if not fourth rate equipment and are running their subwoofers without a high pass filter on the main speakers, people who listen to Yello on Tekton loudspeakers.  There is no way in ---- you can get decent bass out of a system that way. The main weakness of a distributed array system is that there is no way to control time with it. You can not get the impulse from the subwoofers and your main speakers to your ear at exactly the same instant. The result is a mess. But, it really does not matteras none of these people have a way to deal with the problem anyway. They would rather spend their money on cable elevators. 
@mijostyn Both the experts cited have extensive experience in audio, i.e., it’s their profession. Neither are subwoofer salesmen. In both cases, their answers were detailed, and in both cases, they explained pretty carefully -- in Poes’ case, with great detail. In both cases, neither proffered sweeping generalizations about stereo bass, Indeed, the case was pretty well established that in many cases, mono bass was better for room acoustics and might be the only thing hearable, anyway. The information they provided is not susceptible to ad hominem remarks. To be clear, they are not offering arguments that are "pro stereo bass" or "pro mono bass."  They rebut the claim that there is NO such thing as stereo bass and it can NEVER matter. There is, and it can, with all the qualifications provided. I thought that the OP's question was seeking to learn about this topic, so I provided what seemed like good information, rather than just my experience, which is meager.
I run one pair crossed over at 150hz (in front) and one crossed over at 80hz, so I do run stereo, but I doubt it changes a huge amount in the end. I've played with the phase but don't notice any effect. Like @heelid I placed my Swarm at different heights, two lower, one ear level, and one close to ceiling. I experimented with different orientations, and firing toward the wall produced by far the smoothest results. These were very easy to integrate beautifully in my system.
  • mc’s formula is a rip from Floyd Toole’s book, the focus of which is setting up a home theatre, not a HiFi. Read the book and find that the mid wall location works for rectangular rooms. Not all room are rectangular.
  • 80Hz XO and no main roll off is a sure recipe for a mono low end which is fine if you like it. Lots of level, no detail. mc’s Moabs claim 20Hz. Adding 4+ subs from 80Hz is going to create one hell of a bass bump.
  • Floyd states that DSP & EQ can get perfect bass in one location only which is my preference.
  • Phase matters and is both audible and measurable.
@mike_in_nc place a phone call to Duke Lejuene at AudioKinesis who manufactures the Swarm which he'll tell you was based on a theory by Earl Geddes. If you search the web I think there is also a white paper out there by Earl that explains the concept and placement ideas. Duke provided a placement guide based on the Golden Ratio that I dug up somewhere. It is a good start but with some experimentation you can dial it in. He has gained a wealth of information on this topic and is more than willing to share.

I did my own swarm using 10" woofers in sealed boxes. Crossed over at 70 or 100 Hz depending on the mains speakers using an Accuphase F-25 frequency divider. Two woofers firing towards the walls, two woofers firing into the room, one of the four woofers raised to ear level. I put the woofer nearest the corner out of phase by swapping the leads.

Since you have line source ribbons @mijostyn has an interesting set up for that type of arrangement I used with my Acoustat Model 2 speakers. His theory in effect creates a line source for the bass. You can chat with him as well to get more information.
I don't agree with much of anything written on this thread, except for if you like it then you got it correct. On the other hand, bass sub frequencies are felt more than they are heard and you feel directionality in your sinuses, medical science. Proper phasing of the bass frequencies makes a huge difference to the quality of the performance and the power requirements to achieve a great performance. When the bass image is in the same plane as the rest of the image it is magical. Typically this is a achieved with a speaker management system. There are manufacturers who specialize in such equipment. You could say it is all in the science and the knowing.
+1 @audio-union

Totally true but look every obsessed audiophile must choose their obsessions and overlook certain truths to keep it all reeled in or go totally bonkers. Or do they? From the sound of things, perhaps not. 😉
hleeid-
@mike_in_nc  -

Firstly, heed the advice from MC/Noble101/and especially Duke LeJeune.

Secondly, I will divert from phase, crossover frequencies, gain, etc. and instead share my experience regarding subwoofer placement.

I am running four subwoofers (yes, the AudioKinesis Swarm). Two are mounted up high facing the ceiling. Their heights vary with one 12 '' and the other 18 '' below the ceiling.

Duke mentioned in an earlier thread that any subwoofers mounted above the room mid height contribute to the vertical plane, thus further reducing standing bass nodes.

My A/B testing (4 on the floor vs. 2 on the floor with 2 up high) in 2 rooms (10 x 12 minus closets and 12 x 16 living room) at my previous house along with over a year at my new house (also A/B testing between a 13 x 16 vs. a 15 x 19 room) all clearly confirm a noticeable all around improvement.

Hope this is helpful to you!


mike_in_nc, Hope you are paying attention (to the whole thread) because this is typical of what happens every time. A small number of us have experience and knowledge. A whole bunch more talk as if they do. Sift though the chaff to sort out the wheat and you will be fine.
Some additional independent perspective on sub placement for music from a very experienced and knowledgeable source.

For Optimum Sound, How Many Subs, and Where Do You Put ’Em? | News & Views | Ohm Speakers | Custom Audiophile Speakers for Music & Home Theater

" At 80 Hz, it is quite difficult to determine the source of sound. However...."

Hilde 45, Right, of course there is stereo bass. you just can't hear it somewhere below 80 Hz. Sound always has a direction even though in a room it can be confused. Mr McGowan is a snake oil salesman. There are far better experts I care to listen to like Siegfried Linkwitz, Nelson Pass, John Curl, Roger Sanders, Edgar Villchur and groups at Shure, JBL, Harmon Kardin (now Harmon International) and others.
I am not an acoustic engineer however I do design and build my own subwoofers and I have been intensely studying the problem since 1978 when I got my first subwoofer system comprised of first one then two RH Labs subs, a Dalquist DQ-LP1 crossover and Kenwood L07-M amplifiers, SOTA in the day. It became obvious pretty fast that one subwoofer would not do. This is 1978 and many here were still in diapers. An expert by the way is anyone who parrots what you want to hear:-) I was using ESLs so it was not a Radio Shack system. I had an efficiency apartment, a single large room almost perfect for Hi Fi. I use to take a break mid day when everyone was at work and blast myself silly. I digress. Even with two subwoofers there were problems. I could always tell the subwoofers were there when I got them up to realistic levels. It also became obvious quickly that the ESLs sounded better when I got the crossover point up above 100. But, the subwoofers became more obvious. In the end I never got that system to the level it could have performed at with the right electronics but, they were not available then. Everybody else thought it was a dynamite system. I had it in my head that it was possible to make a system that was as gratifying aurally as a live concert and that system was not. Back then, because of the difficulty integrating subwoofers they were considered taboo by the high end establishment. But, none of their systems were as gratifying as a live performance either and to my mind low bass was the biggest problem. I sold that system on leaving Miami and took a break from subs for several years. I was in a regular apartment building in Ohio with wood construction and subwoofers would probably have gotten me lynched. I did discover line source loudspeakers during this period in the form of Acoustat 2+2's and with Krell KMA 100s they sounded marvelous, still no deep low end and still not representative of a live performance. The two main issues were bass and imaging. Every system I had ever heard sounded like the audio version of a TV screen. It was not instruments standing in a space. You could hear the third dimension but it did not encompass the instruments and voices, if this makes any sense. You could hear the size of the venue and items were localized side to side. Just like a TV screen, you can see the size of the space but the third dimension is missing. Even today very few systems are capable of creating the third dimension. Does the band really sound as if it is standing in your room, are you sitting in the venue? People talk about  3 dimensional sound, the third dimension. What they really mean is they can hear the size of the venue like you can see it on a TV screen. Creating 3 dimensional sound requires near perfect imaging and bass. Low bass creates that feeling of air around the instruments. To get the effect you have to have both. Most systems have damaged imaging because the frequency response of the main speakers is not exactly the same. 
 In 1987 I moved back to New England and waiting for me there was a brand new pair of Apogee Divas. The most frustrating speaker I have ever owned. When they were on they were incredible, incredibly fragile and flawed. I destroyed ribbons for 6 years before returning to Acoustat 2+2s. With the Divas I did start up again with subwoofers using the original Velodynes. They were not as good as the old RH labs units. There were no subwoofers on the market I really liked.  In 2000 my Krell preamp got taken out by a lightening strike. I used the insurance money to buy a TacT TCS. I used the multiple channels to biamp the Divas and the bass management system gave me the flexibility to build my own subwoofers and for the first time integrate the subwoofers to the point that they disappeared. Digital signal processing allows you to put the subs where they belong, in corners, and get them matched up in time and phase with the main speakers. Lucky me has a brother with a PhD in aquatic acoustics from MIT. After a lot of scotch, long discussions and some computer math. We came up with the four subwoofer array I still use today 20 years later. I am now working on my fourth and hopefully final version of my subwoofers comprised of four enclosures and 8 12" drivers. I use full range ESLs so the crossover will be in and around 120 Hz 4th or 6th order both high and low pass filters. 
With dynamic loudspeakers that cross out of their woofer below 500 Hz there is no reason to run a crossover this high. 80 Hz is fine but to benefit you have to use a 2 way crossover. Adding a subwoofer in at 40 Hz is IMHO worthless. Very few speakers can project anything under 60 Hz with authority. I always have frequency response specs quoted, " my speaker goes from 25Hz to 50 kHz." Right, at one meter, not to mention that it was already 3 dB down at 25 hz at 1 meter. The idea behind  subwoofers is to project with authority frequencies your main speaker can't down to 20 Hz at 3 meters in a real room, flat. And, to relieve the main speakers from having to reproduce these frequencies which seriously distort everything else the main woofer is doing because of the long excursions they require. 
Subwoofers have to be corrected so that they match the main speakers in phase and time. in order to do this you have to use a digital crossover and correction. It is virtually impossible to do it with an analog crossover and if you think you can do it by ear I have a wonderful piece of property to sell you in Key West. 
OP, my inclination is to always use stereo subwoofers. Most bass is mixed towards the center anyway. 

It’s pretty funny actually when some worry about things like fuse and wire direction and others shrug off bass as suddenly and totally independent of direction unlike all the rest.
There is no such thing as stereo bass below 80 Hz since it’s a fact that humans cannot determine the originating location of any sound deeper than approximately 80 Hz.

<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>

I agree 100% mapman..

It’s not about what we can determine. If you only use the signal from the left of a stereo recording you are leaving out 1/2 of the TOTAL BASS signal. There is a RIGHT side too. There is a left a right and the combination of the TWO.

Mcintosh Preamps
4 outputs; A, B, unswitched and LTE (on the MX120 and C2500)


A - 80hz and down the Rythmik amps (GRs 2 x 12" OB)
B - I run a modified 2496 to split the signal from 80 - 300 hz.
UN switched to the passive XOs for Monitors ONLY (no bass)..
LTE (unused)

When I run just "B" , and turn OFF the monitor amps. LOL There is NO MONO anything... It IS STEREO.. PERIOD.. 80 hz to 300 hz is ALL STEREO.. My ears YOUR ears and anybody else’s ears (as long as they both work pretty good) is gonna hear a stereo signal WITH a center channel to BOOT..

80hz and DOWN has a stereo signal left and right, the TWO are a complete signal with ALL the different BASS tracks laid down. It has NOTHING to do with mono or stereo HEARING. PERIOD!!

JFTR :-)
@mike_in_nc Have you gotten what *you* need from this thread? (MC putting people down and elevating his own genius is, as usual, a freebie.)
It’s nice when people try to help other people. Not so nice when they boast about it and also try to discredit others  in order to claim some sort of monopoly. So the two cancel each other out. See how that works?
 " Phase matters and is both audible and measurable.''                       
   I concur.
@hilde45 -- Thanks for asking; yes, I’ve got what I was looking for (and more :-)). There has been a wide variety of opinions, all worth considering. My thanks to everyone; even if my experience does not mirror yours, I appreciate the time and effort you took to share your thoughts.

I plan to review the entire thread soon and draw my own conclusions.

And yes, probably everything helpful that can be said has been said by now.
@mijostyn -- An interesting story. I must correct you on one point. An expert is more properly defined as, "Anyone 500 miles from home." If one doesn't tell you what you want, another will -- they are a dime a dozen. I know; I was one (in another field).

I also had a TacT (2.2X) and a pair of Apogees (Caliper Signatures), though at different times. Flawed pieces, wonderful in their ways.

With subs, I always have used DSP HP and LP filters and careful adjustment of delay (phase). I have never used a sub without HP-filtering the mains, so can't comment on that idea. In my current setup, no one can localize the subs: bass seems to come from the main speakers, even though the subs are behind (!) the listener, placement needed to fill a room-induced response hole.

Since posting, I've played with a single added mono sub, and it seems to get my setup from 90% to 95%. (Some might not find that enough, but life is getting shorter.) I am mulling over how to adjust its delay -- my electronics aren't set up to do that, but I hope a sub with a continuous phase control will work well enough.
So in spite of terrific testimony you asked for- actual experience- you are playing around with the advice of one who has no actual experience. Par for the course. Oh well.

Moving along... In the interest of helping others following along, here is why crossing over the mains is self-defeating and a bad idea.

The key to excellent low bass is lots of sources of low bass. To the extent they put out low bass this includes the mains. Also more different locations helps, and while subs are almost always very near walls the mains are almost always several feet out. So crossing them over eliminates two very beneficial bass sources. In a realm where numbers matter far more than anything else this is known as a own goal.

Frankly and with all due respect you have no way of saying you got from 90 to 95% for the simple reason you have no idea what 100% sounds like. For sure the one you are taking advice from has no idea, he has never done this.

Once again I am perplexed and vexed at one who specifically asks for one thing- "those who use a DBA"- then turns around and follows one who not only does not use a DBA, but talks down anyone who does.

https://youtu.be/0t2pWUWE1Y8?t=8

Oh well, plenty of others willing to listen and learn. No point wasting any more time here.
@millercarbon  - I trust that my joking exchange about "experts" was not something you took personally. It surely was not aimed at any particular person.
I appreciate your contributions as well as those of others.
Post removed 
I hope a sub with a continuous phase control will work well enough.
Nope.

"All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband."

see http://www.ielogical.com/assets/SubTerrBlues/PhaseControl.png at  ieLogical SubterraneanHomesickBlues for an image of how a continuous phase control functions.
Mike_in_nc, if you get an acoustic measurement mic and program like this one, https://www.parts-express.com/Dayton-Audio-OmniMic-V2-Acoustic-Measurement-System-390-792
You can measure group delays and frequency response. You should be able to adjust your subs perfectly.  I assume with their location you are using a low crossover point. What are you using for your main speakers?
Where you place them depends on your main speakers. Subs always perform best in corners. They are up to 9 dB more efficient which means much less distortion. For point source main speakers four subs in four corners would be ideal but two in corners and two right up against side walls would be a close second. A sub should never be in the middle of the room. That is like running your outboard motor out of water. For maximum dynamics you have to be able to adjust delays so the sound from your subs gets to the listening position at exactly the same time and in phase with the sound from your main speakers. This requires digital bass management. Are you still using a digital crossover? What happened to the 2.2X? Yes, Apogees have a reputation they do not deserve. They were capable of amazing sound on occasion but their fragility disqualifies them. Magnepans are a much better design from a reliability standpoint and if a ribbon tweeter blows (which they do rather frequently) Magnepan has a great tweeter replacement program and if you can use a screw driver you can change the tweeter. Apogee went belly up for a reason. 
Don't know if you are familiar with  Earl Geddes?

You should check him out and read the several white papers regarding subs and other stuff

http://www.gedlee.com/Papers/papers.aspx

@danager -- Thanks for the link to the Geddes papers. I surely know the name, but I don’t think I’ve read anything of his.

@mijostyn -- I use Janszen Valentina P8 main speakers. In my basement room, they respond well to 20 Hz, but a room-induced hole at 40Hz necessitates subs. As you say, subs project more authority at LF, even though SPL may be the same. The two Fathom F112 subs are in the rear corners, next to floor-to-ceiling bass traps. That is the spot in which the 40Hz hole can be filled most reliably. I use digital crossovers at 80 Hz, mostly, done by an Anthem STR Preamp, far better sounding than the Classé CP-800 it replaced.

I used to adjust sub delays with an OmniMic2, but that broke. (Lesson learned: do not position measurement mics by grasping the wand and moving it.) In a stroke of fortune, I won a UMIC-2, so I am learning REW, which appears quite capable, if complex.

I sent the TacT 2.2X to a mod shop in southern California to have the SRC replaced with a more modern one and some other components swapped. Unfortunately, the proprietor never finished the work and never returned the unit (which five years ago, was in pieces in a storage locker somewhere). The Anthem is one of the newest generation of audio DSP devices, and unlike the TacT, and very unlike the Classé, I don’t hear any artifacts when engaging the DSP.
     I completely agree that Duke Lejeune of Audio Kinesis is the most knowledgeable. best and most credible source on these forums about all things concerning 4-sub DBAs. He's always willing to assist and has probably forgotten more about proper 4-sub DBA systems and setup, as well as how bass sound waves behave in domestic sized rooms, than I've ever known. He often chimes in on threads like this but, unfortunately, not this time.   I agree with clio09 that you should call or email Duke at Audio Kinesis for the best advice.  Best wishes and enjoy, you're going to be amazed once your DBA is setup properly.
     Here's a little history on my experiences with the DBA concept for those interested:
      I actually first learned of this exceptionally good bass concept by chance.  Over 10yrs ago, I was just looking for the best way to supplement the bass performance of my combo music and HT system that utilized Magnepans as mains. I had called Audio Kinesis about another matter, talked to Duke's associate, James Romeyn, and we got talking and emailing about my system goals.   He suggested either of their two 4-sub DBA versions (the AK Swarm or Debra) as a very good bass solution for my Magnepans.  
     This was the first time I'd heard about the 4-sub DBA concept and it really piqued my interest. I dove into reading all I could find on-line about the subject, beginning with Earl Geddess's PHD Thesis on the concept, his subsequent White Papers on it, articles on multiple subs by Floyd Toole and Harman's Todd Welti as well as several others.
      At the time, I was planning on buying a pair of high quality subs such as JL Audio, REL or Vandersteen for up to about $6K.  But I decided to give the AK Debra a try instead since this whole scientifically tested and proven concept of the 4-sub DBA made so much logical sense to me, I was very interested and curious about its practical performance, I felt a bit adventurous and wanted to try something new, interesting and nonconventional.  
     Anyway, I was amazed, and still am a decade later, at how well this DBA concept performs in my 23'x16' room.  It took half a day to properly set it up but I didn't care once I heard and felt the results. It performs way beyond my expectations on both music and HT and is definitely the single best upgrade I've ever made to my system during my 50+ yrs in this hobby.   Hyperbole?  Maybe, but this concept is outstanding and I know it will work equally well with any main speakers I care to utilize now or in the future.

Tim  
@noble101--  I am acquainted with Duke through other channels and have had some information from him since posting. I do agree about his extensive knowledge on this topic, through setting up systems in many situations, as well as design work. I have observed Duke help users like me, who are unlikely to be buying a Swarm from him, and I've concluded that he's not just well informed, but one of the real gentlemen in our field.

I also like hearing peoples' experiences in their own systems; yet, it is not always as possible to generalize from them as it is from those of someone who's done the same thing in varied circumstances.
My name has come up a few times in this thread.  I don't really see how to participate without either taking a side in an emotionally-charged "us versus them" thing that I want no part of, or turning into a self-appointed thread nanny.  Until I figure that out, I'll just tip my virtual hat to @mike_in_nc for taking the high road.  

Duke  
distributed multisub system manufacturer since 2006
Hello Duke,

Completely understandable.

Thanks,
  Tim
Understandable.   I feel the same way quite often. 
Great Mike, I recommend the Anthem all the time along with the Trinnov Amethyst and the DEQX units. I need to look into the MiniDSP units as they are even less expensive and seem capable. 

I have not heard your speakers but I am a big ESL fan. They are point source. I would add two more JL subs in the front corners. 80 Hz is fine, 24 to 48 dB/oct. 

The TacT has very proprietary circuitry. You really can not do anything with it. Shame what happened to it. I plan on getting the Trinnov but I won't like losing the dynamic loudness programming. 

If you can create separate target curves for the channels work on EQing the speakers so their frequency response matches perfectly 100 Hz to 10 kHz. You will like what happens to your imaging. You have to look at it with your measurement system. The room control programming will not do this. It is well worth the work. 
millercarbon I am afraid you are dead wrong in asserting time and phase do not matter. You are saying this because you have no control over them. If you did you would be piping a different story. Out of phase and time destroys bass transients like kick drums and thumbing bass guitar strings. If you had the ability to control phase and time you would know otherwise. 
@mijostyn - What ruled the Amethyst out for me was that it has a fan, which a couple of reviewers have reported hearing. In my very quiet room (~26 dBA SPL), any fan noise would be obtrusive. Since Trinnov is reputed to have the very best RCS, I wrote to them and asked if they might release a fanless version. They said no.

"All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband."

Yes, that’s because they are really delay controls, and a given delay is equivalent to different fractions of a wavelength (phase) at different frequencies. But as far as I can see, delay is what’s needed to align speakers that are different distances from the listener (and might have different latencies).

At least, that's my understanding of the situation.