Which is better for a DBA (Swarm); powered subs or unpowered?

Hi cheeg, 
let me relay my subwoofer experience to you and a recommendation to use your $1K wisely. 

I have successfully and seamlessly tuned subwoofers in my car and home systems.  
I started out with one decent sub at home and it worked really well and two worked slightly better and was very successful. 
If you want the best bang for your dollar (2) Rythmik L12 ($559 each, shipped) will sound excellent.  Someone I know has this setup and it is impressive in a high end 2-channel setup. 
Second best would be (2) SVS SB1000 subs.  

Start by placing them just to the outside of each speaker slightly away from the corner and back wall. you can adjust position once you have broken them in.  

"tune" one sub at a time- tune the right sub to your system noting the best crossover that blends and doesn't cause any peaks or notes that are too loud.  Then adjust the phase by "timing" it to your main speakers.  Play some repetitive bass beats and adjust the phase for the most focused, most powerful notes.  Adjust level to blend.  
With the first sub playing (as a system) add the second sub to the "system" and repeat the steps for crossover, phase and level.  
Now run with this for several weeks and adjust only the level of each sub.   Take notes and write down all of your settings and when you make adjustments in the future.  
After that the subs will be worn in and likely sound louder and slightly different.  Adjust as needed to make the sound smooth.  
This has worked very well for me.  
peace and good luck 

@noble100 Thanks, Tim, for your detailed responses -- I feel like I'm getting a free education!  (and I admit, my head does hurt a little)
@ieales Thanks for the additional notes on All-Pass phase control.  I think I just need to listen to one now, and see if the theory matches reality. 

Hello cheeg,

     I’m just going to complete my last post as I previously stated:

Lastly, I wanted to provide you with some easily verifiable facts that should allow you to more easily recognize the bad information you’ve been given already on this thread as well as any that may be given to you in the future. But I have a meeting I need to attend right now, so I’ll give this info to you later on another post as soon as I can. Continuing:

1. Humans cannot localize sounds (determine exactly where they are originating from) with frequencies below about 80 Hz, which includes all the audible deep bass tones down to the audible limit of 20 Hz.  We are increasingly adept at localizing sounds as their frequencies increase from above about 80 Hz to the generally accepted audible upper limit of high pitched sounds of 20,000 Hz.

2.  Recording engineers, of course, have known about this fact since at least about 1950 and is the main reason they’ve been summing all bass tones, with frequencies at or below about 100 Hz, to mono ever since.
     Some individuals will inevitably deny this is true.  I usually just ask them to name a single commercially available recording that contains stereo bass below 100 Hz..  I’ve never been able to name a single example and neither has anyone else.  

3.  It follows logically from the above 2 facts that the reproduction of true stereo deep bass on a home audio system is not possible, as long as commercially available LPs and CDs are the source material played.      Therefore, it makes little sense to position a left and right sub near each of your left and right main speakers.  It’s best just to operate all subs in mono and position them where the bass sounds the best using the crawl method.
 4.  Bass sound waves behave very differently in a given room than midrange and treble sound waves behave.  This is mainly due to the facts that bass sound waves are omnidirectional and very long while midrange and treble sound waves are highly directional and much shorter.  The length of a frsound sound wave is directly and inversely related to its frequency.  The lower the frequency, the longer the corresponding sound wave and the higher the frequency, the shorter the corresponding sound wave.  It’s a continuum with the lowest audible frequency of 20 Hz having a sound wave length of about 56 feet and the highest audible frequency of 20,000 Hz having a sound wave length of a fraction of an inch.
   So, the long omnidirectional bass sound waves are launched into the room from the sub in many directions and, if the frequency is deep and long enough, each sound wave could exceed any single dimension of the room.  Each of these bass sound waves will continue to move out into the room until it meets a room boundary (floor, ceiling or wall), reflect or bounce of  this surface and continue moving in the reflected direction until it meets another room boundary and is reflected once again.  This process continues until the sound wave either collides with another reflected sound wave (causing a standing wave) or it runs out of energy.  

5.  Each time any of these moving and reflecting bass sound waves collide with another sound wave, what’s called as bass standing wave results at the specific room location the collision occurs and we perceive the bass at these exact spots as a bass peak, dip or null.
     If a 4-sub DBA system is utilized, our brains sum and average the bass by frequency and no bass peaks, dips or nulls are perceived in the room.  If only a single sub or pair of subs are utilized, the crawl method can be utilized to ensure the designated listening seat is not located near a bass standing wave.

6.  Interestingly, it’s been proven that a complete,full cycle and full length bass sound wave must exist and be detected by our ears prior to our brains can process it and create the perception of a bass sound at the appropriate depth of pitch in the room 
     If the bass is deep enough that the sound wave exceeds any of the room’s dimensions, this means the sound wave must reflect off at least one room boundary before we perceive a bass sound or tone at all.

7.  Our ears and brains are much less sensitive to the arrival time of bass frequency sound waves than midrange and treble frequency sound waves.  I believe heaudio123 correctly mentioned this earlier on a post.  I’m not absolutely certain, but I believe as long as bas sound waves reach our ears within about one fifth of a second of each other, our brains process these sound waves as related and parts of a single sound or tone.       This means the sound waves from all 4 subs in a swarm definitely do not need to arrive at one’s ears at the same time.  There’s no strict timing requirements or resultant “smearing” involved with multi-sub bass reproduction.

8.  Of course, the much shorter and highly directional midrange and treble sound waves also reflect or bounce off room boundaries (floor,ceiling, walls) until they collide with one another or run out of energy, just as bass sound waves do.        It’s much more important that the main speakers are precisely positioned in the room, and in relation to one’s ears at the designated listening seat, so that the direct midrange and treble sound waves from the speakers arrive at the listener’s ears first, before any reflected sound waves do to ensure optimum performance, imaging and quality.  However, it’s much easier to useroom treatments to absorb or diffuse first reflection points of midrange and treble sound than it is to do the same on first reflection points of bass sound waves.       There’s also no performance, imaging or quality concerns dependent on the direct bass sound waves from any specific, or combination of subs, arriving at the listener’ ears first.  Any midrange and treble sound wave collisions resulting in midrange and treble peaks, dips and nulls at specific locations in the room, are typically not a big issue since we tend to perceive these as an ‘airy’ quality to the overall sound in the room.

Hope these facts helped your understanding,

Tim

An All-Pass phase control alters the phase relationship between the frequencies passing though it relative to one another. Hence, the phase relationship to the mains varies by frequency.

Moving the sub physically or with a delay does not alter the phase relationship of the reproduced frequencies relative to one another. The phase relationship to the mains is altered by a constant for the frequency in question.

Moving a sub changes the room and main interaction. A delay or phase control does not change room interaction, just mains.

Mixing on headphones is a modern abomination. Headphones were only used to set up a cue mix. Ditto egregious nearfields like Genelecs.

You are getting a lot of good advice here and in my opinion I don't think any of it is bad. In the end you have to sort through everything and decide for yourself what will work best for you. There are many ways to implement a subwoofer or distributed array. Unfortunately, in each of these threads there is one particular member that shows up with their it's my way or the highway monologue. This one is no different.

Let's get to the point. You have $1000 for your budget if I read correctly. That's not going to get you very far in the DBA religion. Heck, my DBA cost $4k and I built my own speakers. Unless you increase your budget you are better off searching the forums for information on good powered subwoofers, then search the listings to see if you can find 2 or 3 used ones that fit within your budget. At a minimum you will get an idea on what multiple subs can do for your system, as well as learn the ins and outs of setting them up which is critical.

If you did have a $3k or so budget I'd point you to Duke Lejeune of Audiokinesis. His turnkey DBA is the most thought through commercially available solution. More importantly though, I think you should look him up and contact him directly, as he is a wealth of information on this topic and speaker building in general.

Hello cheeg,

     I just read this entire thread and I feel bad for you. The reason is because you’ve been given a lot of bad advice and false information. The majority of this bad advice and false info was delivered by ieales, clearthink and audiorusty. I’m assuming this bad info was due to a lack of knowledge and not intentional but I can’t be certain. You did an admirable job of trying to make sense of it but I think it’s best just to disregard their posts entirely, hit reset and start anew.
It seems like you were already aware of the 4-sub Distributed bass array (DBA) concept. You somehow had learned about how it provides near state of the art bass performance in virtually any room and integrates seamlessly with any pair of main speakers. This is has all been scientifically proven and can be relied upon as valid.      I’ve been thoroughly enjoying an Audio Kinesis Debra 4-sub DBA system in my room for over 5 years now for both music and HT in my 21’x14’ room. The bass is fast, smooth, detailed, as powerful and dynamic as the source material calls for down to 20 Hz and this high quality bass integrates seamlessly with my fast, smooth and detailed main speakers.
    The truth is that a 4-sub DBA system will definitely provide this near state of the art bass reproduction in your room and seamlessly integrate with your main speakers. This will remain true whether you decide to use 4 passive subs powered and controlled by the Dayton SA1000 or 4 self-amplified subs.
      Having the capacity to adjust volume, crossover frequency and phase on each sub individually obviously gives more bass system control. In practice, however, you’ll rarely, if ever, need this level of control and it makes initial setup much more difficult.
     Lastly, I wanted to provide you with some easily verifiable facts that should allow you to more easily recognize the bad information you’ve been given already on this thread as well as any that may be given to you in the future. But I have a meeting I need to attend right now, so I’ll give this info to you later on another post as soon as I can.

Tim

@heaudio123 I could imagine that may be true if the wavefront timing is dealt with, but I don’t see how an amp that adjusts all 4 subs as a single group could get the wavefront timing right if the subs weren’t  equidistant from the listener. I can see how the array would work if each sub has its own time delay setting. Am I missing something here? Thanks again!

@ieales Thanks for the clarification, but I think I understood you the first time.  As I understand it, "fat" means that the wavefronts from the four subs reach your ears at different times, in the case where they are not equidistant to the listener.  That is not the sound I want.  However, even if you DO correct that (either by spacing the subs the same distance from the listening position or by inserting time delays to get the same effect), the phase will still be an issue, since (as @heaudio123 noted) the phase is a function of the frequency.  Since the wavelength of a bass note varies by a factor of three in the range from 20 to 80Hz, that means (someone please correct me if I'm wrong) that the phase could be perfect for the listener at 20 Hz, 40Hz and 80Hz, but off by as much as 180deg for the frequencies in between.  Having said that, I don't know how AUDIBLE this is, and I'd rather not waste my discretionary spending on something that fixes one of these issues (like time delay) at the expense of the other issue (phase).  And again, thanks for chiming in; I hopefully am learning enough to be dangerous...

Distributed bass arrays do not result in "fat" bass.

By FAT I mean that unless time correction is implemented the separate sub signals will arrive spread over several milliseconds.

The beater whack on a bass drum will arrive asynchronously to the head resonance, which will arrive multiple times. Depending on the room, subs and positioning, some may arrive before the beater and some after.

IMO, it's as unlistenable as MP3 which are only acceptable in mono.

Perhaps it's because I sat so long in a recording engineer's chair, but there is ONE spot that has the correct image. Period. I never worked in any studio where the monitors, if they were time aligned, were not aligned at the engineers seat. NO STUDIO EVER had distributed bass.

Thanks for all the new responses! Let's see if I have this right:

@ieales doesn't like Dayton subs b/c they have "no delay so there is time smearing. What you end up with is a fat, detail-less low end pillow."

@b_limo seems to agree with @ieales, at least to the extent that "with the dayton amp, you aren’t able to adjust [the phase of] each one independent from the other", which is needed if the subs aren't equally spaced from the listening position. Makes sense.

@clearthink also seems to agree with @ieales, unless the listener likes loud, thumping bass (I do not, unless I'm wasted and listening to loud, poorly produced classic rock (which does happen, but not as often as I'd like).

@audiorusty seems to lean toward powered subs over one bass amp, because they have more individual control. But he cautions that DBAs are more easy to integrate, since you only have one amp to adjust. My response would be that I don't enjoy spending time tweaking my speaker settings, but if that's what I need to do to get what I consider to be "good sound", I will put up with it. Score one for powered subs. (since you asked, my taste is generally 60's-70's folk, prog rock and classic rock at moderate volume (85-90 dB) for enjoyment, and classical at lower volume for background while working or going to sleep (but I still want it to sound clean). Not so much jazz.

@heaudio has a LOT of good info regarding the difference between getting the time delays right vs getting the phase right, and I have sooooo many questions. He seems to think you can get this right with a well implemented DBA, but I'm not sure how. If he has time to discuss it more, I'd love to know how to text him (or email, phone, whatever).

@millercarbon initially gave some very useful input, and seems to recommend Dayton(s) in a DBA, but my last round of questions scared him away and he has moved on. Nevertheless, I thank him again for all his advice.

So, after all these responses, I have a lot better understanding of the issues, but am no closer to deciding whether I should buy a Dayton and cheap passives, or buy two better quality actives and save my pennies for 2 more.  Maybe the answer is "it doesn't matter", but I really hate that answer... Thanks for hearing me out.  Peace, and enjoy the music!

A phase control does not adjust the timing. If you are 10 feet from 1 sub and 5 from another there is a time delta between the arrival of the two wavefronts.

Some people like this fat bass. 

All-Pass phase controls operate at only a single frequency with varying degrees of shift across the passband.
For example, the control 0° position 90° frequency could be 2.5KHz and the 180° position 90° frequency could be < 5Hz.
See  http://www.ielogical.com/assets/SubTerrBlues/PhaseControl.png for how an All-Pass phase control affects frequency.  http://www.ielogical.com/assets/SubTerrBlues covers integrating bass properly.

Note that the phase is not constant across the sub's range. To have constant phase, you must have a delay line or physically move the sub.

How loud do you like to listen? Do you listen to a lot of rock and want gut thumping low end or do you prefer classical or jazz and are looking to just fill in the bottom end?

With powered subs you need proximity to AC and you will need interconnects, unless your subs have wireless calability. If you are using RCA connectors and have some long runs over 10' to 15' that could be an issue. If your system is balanced then long IC runs should be fine. With passive subs you will need speaker cable and if you require big power and a long run then you may need to use heavier gauge wire up to 12 awg maybe even 10 awg.

If you like it loud, a good powered sub will offer better speaker protection. With passive subs and a separate amp you will more than likely get none, so if you hard clip your amp it will probably be re-cone city. The rule of thumb (at least it use to be) is to use an amp that is capable of twice the output power of the speakers maximum handling capacity. So if your subs can handle 200 watts @8ohms then you want an amp that can deliver 400 watts @8ohms and so on.

With powered subs you will have more individual control, which for some will be a good thing and for others not so much. A dba system itself makes integrating easier but as the room gets larger control becomes more important. It will be dependent on how sensitive you are to time domaine issues. You should be fine either way with your room.

Some subs (I think REL is one) allow you to wire them in a way that reduces the burden on the main amp to cover frequencies below a set cutoff, like 80Hz. Does anyone know if that is possible with the Dayton?

Almost all powered subs and I believe this is also true of the Dayton amp, only come equipped with a low pass filter, which allows you to set the frequency cutoff point of the signal that is sent to the subs.  You would need an actual crossover to do reduce the burden on your main amp. 

is it advisable to use all the same brand/model of subs in the swarm, or does it not matter?

There is not a lot of detail when you get below 80hz so in my opinion the subs do not need to match. I use three different brands of subs in my 4 sub dba with no issues.

what is REW? 

I believe REW is a type or brand of room analysis software.

Is it possible to run 4 passive subs off of one amplifier, say like a Crown XLS series? 

Yes as long it has enough power and can drive the impedance load that the speakers are presenting. The load will vary depending upon how the cabinets are interconnected.

ieales"DBA subs give a time incoherent mess everywhere."

Of course this is actually valid, verifiable, and repeatable how ever there are "lead heads" who love lots and lots of bass they like to "pump it up" so of course for them they need many sub woofers and for them it is best to have a variety of them mixing in 12 inch subs with 15 inch subs with smaller subs and a mix of low quality amplifiers often "Class D" which is really "Class Deficient"!

Do you not have to adjust the phase of each sub independently? I wouldn’t think that with four subs in different locations, possibly pointing in different directions, that you would want the same phase or even the same volume levels across all of them equally...

I gather that with the dayton amp, you aren’t able to adjust each one independent from the other (?)  

Remember, 2 properly positioned subs will provide very good bass at your designated listening position only, but not throughout your entire room.

DBA subs give a time incoherent mess everywhere.

The Dayton has no delay so there is time smearing. What you end up with is a fat, detail-less low end pillow.

For music, two subs correctly positioned relative to the mains for the main listening position are the most realistic.

Of course, if the mains are an incoherent mess spraying sound all over the room, who cares?

Is it possible to run 4 passive subs off of one amplifier, say like a Crown XLS series? Run in series or parallel?

I don't see why not. It appears the power ratings spec down to a 2 ohm load. If you run two 8 ohm speakers in parallel on each channel, or two 4 ohm speakers in series/parallel per channel you will attain a 4 ohm load.
I use an Acoustat TNT-200 to power my passive sub boxes. I have four 8 ohm speakers per channel in series parallel. A total of 8 subs in the system.

My set up is a bit different in that I triamp using a 3-way active crossover. My two-way ribbon speakers use an external passive crossover and I replaced it with the active. Under 100 Hz to the subs, 100 - 650 Hz to the bass/mid ribbon, and over 650 Hz to the mid/treble ribbon. A push-pull tube amp for the bass/mids and an OTL tube amp for the mid/treble.

mc -- thanks for leading me through that photo. Sorry to belabor the point, but I'm still not sure what's going on (if you'd rather do this offline, please let me know how I can DM you). It looks like you have two Dayton amps powering 4 subs, and a powered Talon sub that gets its signal from a bypass through the Daytons (presumably the cable that's half red and half copper colored is going out to that sub); I assume that's because you didn't have another line output available on the Melody. I think one channel (white POC) is going to one Dayton and another (red POC) going to the other. I'm not sure why you use 2 Daytons; does one not have enough power for 4 subs? If so, does that mean you're only powering 2 subs with each Dayton, and feeding them the same (white) channel on both their L and R inputs? Also, it looks to me like the two silver cables with the 3 bands are going from a two channel Melody output to an input on what looks like a CD player -- or is that an output from the CD to a Melody input? Again, sorry for all the questions -- they really should have flunked me out of engineering...

I agree with @noble100  that the bass improvement when going from one sub to two is on the order of twice as good.  I stopped at two for now and I am very happy with what I hear but I know the folks with four properly set-up subs mostly report even further improvements.   If you could wait a bit until you can expand your budget the (award winning) swarm system by Audio Kinesis ($3,200 for all) seems bargain priced for what you get and may actually be less expensive than piecing something together and then upgrading.

@millercarbon thanks for the details -- you have a beautiful system, but I couldn't tell from the photos if you were using both powered and unpowered. Do you run your powered subs through the sub amp, too? If so, what is the advantage?

Look real close at image #9 you will see a dirt cheap POC IC going from the Melody to the Dayton amps. Its fake gold with a spring and white and red o-rings. A real POC. Immediately to the left of it on the Dayton is a clear plastic IC with a silver RCA with 3 black rings, and just below it you can just barely make out the other one with 3 red rings. The signal from the Melody comes into both Dayton amps, then uses the Daytons built-in bypass to connect to the additional Talon Roc powered sub.

So its "through" the Dayton but Daisy-chained not actually running through any amplification, EQ or level circuits. 

Some subs (I think REL is one) allow you to wire them in a way that reduces the burden on the main amp to cover frequencies below a set cutoff, like 80Hz. Does anyone know if that is possible with the Dayton?

This is another one where you'll hear guys who love tech more than music tell you all the wonderful tech reasons to do this. There are wonderful tech sounding reasons for doing all sorts of things that don't really work out well in practice and this is one of them.

The idea is supposed to be that by relieving the amp and speakers of the majority of energy which is bass that you will get improved detail and a greater sense of ease, and on and on, probably cure cancer, almost certainly cure cancer if a REL is in there somewhere. Amazing sub, REL. Don't even need a DBA all you need is a REL. Yeah I am being super sarcastic.

Because, always left out of the equation is the detail you lose running that signal through the crossover circuit, and extra interconnects, and how its all affected by the quality of the power and on and on and on. They conveniently leave all that out.

What they also leave out and this one is even bigger, is the whole reason we're doing DBA is to take advantage of the superiority of having a lot of different bass sources. Of which your two main speakers are two additional sources. So you shoot yourself in the foot with the bass, add extra stuff in the signal path, don't get the improvement they promised, and actually make the bass worse in the process.

Great responses -- thank you!
@spenav what is REW?
@spenav and @noble 100 How would you send the wireless signal to the sub? Wouldn't that eliminate the benefit of reducing the main amp's need to drive the lows?
@noble100 I may hit you up for that series/parallel wiring info; is the benefit related to controlling the impedance of the swarm, or something else?
@millercarbon thanks for the details -- you have a beautiful system, but I couldn't tell from the photos if you were using both powered and unpowered. Do you run your powered subs through the sub amp, too? If so, what is the advantage?
@mofojo I'm pretty sure you can run 4 passives off of a Dayton, but I'm not sure about the Crown. Maybe someone else who knows can respond.

Some subs (I think REL is one) allow you to wire them in a way that reduces the burden on the main amp to cover frequencies below a set cutoff, like 80Hz. Does anyone know if that is possible with the Dayton? Finally (thanks for your patience), is it advisable to use all the same brand/model of subs in the swarm, or does it not matter?

cheeg- sorry, my pager battery died-

Starting with $1000, am I better off buying two used powered subs, three less expensive used powered subs, or a subwoofer amp (eg Dayton SA1000) and two (less expensive) used unpowered subs? What is the advantage of having a discrete subwoofer amp?

Either way will work and depending on your situation and deals you might come across which is better could go either way. Either one will allow you to start small and build. So good question.

That said, the Dayton approach has several advantages:

Reliability- powered subs typically use plate amps that are not all that reliable. Dayton and Parts Express are more reliable.

Quality- having the amp separate from the speakers makes it easier for you to select higher quality subs. With powered subs a little bit of each purchase goes into the amp. You’re buying amps over and over again. So you can get a little more speaker for your money this way. Also if you DIY you can get a lot more speaker this way.

Flexibility- The Dayton has just the EQ, level, filter, boost, and phase control that you need. Most powered subs will have these too. But buying on a budget maybe not all or quite as good. With the Dayton no worries. Also with the Dayton when you get more subs you’ll be able to experiment with connecting subs in series or parallel to change the impedance which changes the sound, which turns out to be a pretty good advantage.

Convenience- With the Dayton you set levels all at once. One interconnect. One power cord.

Main thing to keep in mind when going this route is the unbelievable benefits of a DBA really have more to do with the number of subs than anything else. So you will probably get better results faster with 4 cheap subs than a Dayton and one or two better subs. In the long run though as long as you are sure to complete this with 4 (or more- I have 5 and the 5th really did make it better) then I think the Dayton is the way to go.

Maybe even bigger thing to keep in mind is you just can’t imagine how good the bass will be. Its something you have to experience to believe. Maybe not even then. Had mine a year now and it still amazes me.

Oh, and doing one does not rule out the other. You can do both. Have you seen my system?
https://systems.audiogon.com/systems/8367

Is it possible to run 4 passive subs off of one amplifier, say like a Crown XLS series? Run in series or parallel? 

Ha!, Very nice spinaker01. I hope advice from a fellow member of the 4-sub DBA Club (AK Debra system) will suffice for cheeg, instead.
My opinion is that 2 subs will sound and perform about twice as well as 1 sub and 4 subs in a DBA configuration will perform about twice as well as 2.subs. I first heard this opinion from the owner of Audio Kinesis, Duke Lejeune, but I’ve verified it’s accuracy via my own experiences in my own system and 21’x14’ room.
You’ll clearly notice the advantages of using multiple subs beginning with the use of 2 properly positioned subs and these advantages have been proven to be optimized in virtually any room and with any pair of main speakers with the use of 4 properly positioned subs. The advantages you’ll clearly notice are the bass being faster, smoother, more powerful and dynamic, more detailed and better integrated with your main speakers. The main difference between using 4 subs rather than 2 subs is that 4 subs will provide improved bass performance throughout the entire room while 2 subs will restrict this improved bass performance to a single designated listening position or seat. You’ll also notice the bass improves in its speed, smoothness, power and dynamics, detail, sense of effortlessness and integration with your main speakers as you move from using 2 subs to using 4.
No matter how many subs you choose to initially or eventually use, it’s very important you have controls to adjust the following: a regular Volume control for level, a Crossover Frequency for adjusting the frequency below which the subs become active and a Continuously Variable Phase control for fine tuning the sub‘s phase from completely in-phase to inverted phase and all points inbetween. If you use self-amplified subs, all 3 of these controls are required and need to be set on each sub individually. If you use a Dayton SA1000 sub amp/control unit with 2-4 passive (not self-amplified) subs, all 3 controls are set once on the Dayton SA1000 sub amp/control unit for all connected subs as a group.
If budget is a concern, I suggest you start by buying the DaytonSA1000 sub amp/control unit and 2 either used passive subs or 2 new passive subs from Parts Express or Monoprice. If you’re handy, you could save even more $ by building your own subs with kits or individual parts from Parts Express, Monoprice or any other source.
I just recommend you choose subs of reasonable size and weight, with a minimum of 10” drivers, rated bass extension down to at least 20 Hz and 4 ohm impedance if you can. Remember, 2 properly positioned subs will provide very good bass at your designated listening position only, but not throughout your entire room. However, it’s also possible, in some rooms, that 3 properly positioned subs will perform as well as 4 With very good bass throughout your entire room.
There’s also a unique wiring method you need to use for best results called series/parallel which I can detail later if you decide to go with this option.
Another option is to use wireless self-amplified subs such as Syzygy or another brand. I also know the SVS SB-1000 self-amplified subs are very good subs at a great price of $500/each which could work very well for you as either a pair or a swarm.

Tim

You probably would be better off with powered subs IMHO. The second hand market is a good place to start. You can get a used Martin Logan Grotto i for less than $500 on EBay. If one amp goes bad in a powered Swarm, you don’t have to shut down the whole system to get it fixed. Use wireless transmission to daisy chain them and you would be able to move them around without the limitation of a cord. Get familiar with REW to be able to set them up properly and sky is your limit. 

Millercarbon.....paging Millercarbon....aisle 3 for DBA dissertation