Subwoofer recommendation for swarm

I want to create my own subwoofer swarm am looking for reasonably priced units, want to stay under 2K.  Recommendations welcome.  My room is 20' L X 13.5' W X 9' H. 
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One feature you might want to look for is a steep rolloff slope for the variable-frequency lowpass filter. You want to rapidly roll off the top end of any subs that are closer to you than the main speakers are, so that they don’t pass upper bass/lower midrange energy loud enough to give away their presence. (The ear tends to localize by arrival time, which is why this matters for subs closer than the main speakers.) If that’s not possible, then use a lower low-pass frequency setting for subs that are closer to you than the main speakers, and a higher low-pass frequency setting for subs that are equal distance or farther away than the main speakers.

In case you decide to "roll your own", here is the amplifier I use, which has a 24 dB per octave lowpass filter:

Parts Express offers several passive subwoofer kits (you’d have to paint or othewise finish the enclosures after assembly). 

So does I designed three high-output 12" subwoofers which are offered as kits by Leland Crooks of Speaker Hardware. (I don’t get any royalties or other kick-backs from the kit sales, just so you know.) Four of these plus the Dayton Audio amp would strain your $2k budget, but I mention them just in case.

Swarm guy
Would something like a miniDSP with 4 output channels fit the bill?

Duke - Do you recommend trying to feed a stereo signal to the swarm? Summing left and right to mono? Only running the right channel to all four subs? Something else?
My DIY Swarm was built earlier this year. Based on Duke's and using two of the Dayton amps he recommended above, together with four 10" Morel woofers from Parts-Express the total cost excluding Rosewood veneer was not much more than $2k. You could easily build it for less than $2k simply by using one Dayton amp instead of two. Two are not really necessary, the extra amp just adds more flexibility. Based on my experience the first thing I would cut to save money is the extra amp.

Even if you have your own shop and skills and materials its still hard to beat the kits. I have all that, including the MDF just sitting there, yet still went with the kit. Savings in sawdust alone makes it worth it. Although if you have the time and inclination to dig into it you might be able to learn enough to make your own enclosures custom for your particular situation, which might repeat might be worth it. 

I would also seriously consider anything Duke has to say on the subject. He was a huge help to me and always recommending whatever would work best for me, never pushing anything in particular even though it would have been much easier to just try and sell me his Swarm.

There are a lot of options. By far the most significant decision you can make is the one to go with a distributed bass array. The DBA is so inherently better than everything else its hard to believe until you experience it first hand. 

How do you distribute the subs? Can they all be on the same side of the room or on just two walls?
mkgus, a DSP designed for bass management will work fine. The subs can be placed anywhere in the room. I think against a wall or in a corner is best as this placement increases efficiency. I would put the two front subs in the front corners and the others anywhere. Ideally if your crossover is low enough a mono bass channel is optimal. With a swarm system I think it behooves you to use a cross over below 80 Hz and as Duke suggests use a steep curve, 24 db/octave or higher. If you are using a DSP bass management system I have seen crossovers as steep as 80 dB per octave which I actually think is too steep. The speakers do not blend well. 48 dB/oct is the steepest I would use. If you put them all along the front wall they may actually overwhelm your speakers. It is always OK to experiment. Just leave yourself extra wire until you are sure of the placement. 
How do you distribute the subs? Can they all be on the same side of the room or on just two walls?

Well mine are just on two walls and work just great. I've moved them around experimenting a bit but with my system its almost impossible to have one on each wall. Well I could by putting one or two up near the ceiling! Which by the way according to DBA theory would be better. In practice however mine works so well I just can't see going to much effort trying to improve it.

Mine are on the left and right walls, each one a different distance from the corner. Plus I have one powered sub almost exactly in the front left corner. None of these is localizable, to the extent I often times have to touch the cone to be sure a sub is really working. Really low bass is amazing- it sometimes is so diffuse you feel the whole room immersed in it, yet other times its imaging the drum or bass guitar precisely on the stage right along with everything else. Seamless.

Tim aka noble_100 has a room almost identical to mine (one foot less each way) and he went through a very definite procedure to locate each one, which as I recall resulted in one very near each corner. 

Remember speaker location is only half the equation. The other half is listener location. The other 50% as Yogi Berra might say is having four.

I'm looking at buying the 10 inch subwoofer kit with the external Dayton amplifier.  Which driver is recommended, is the one that comes with the kit a good one?  The price is nice the stock way but I have wiggle some room for better drivers. I'm planning to buy 4 of them. Any thoughts? 
Bigger is better. Bigger magnet, bigger voice coil, better faster response. Bigger cone, more bass. Bigger box, well you get the idea bigger is better.

That said, impedance and efficiency are important. Especially if you will be driving all 4 off the one amp, then you want to be sure it will be able to drive them all at once. With mine, thanks to having two amps I was able to compare them wired for 4, 8 and 16 ohms. While they deliver more power into 4 the bass was much better by far at 16. According to Duke none of his customers who have compared have preferred 4 ohm. Something to keep in mind when comparing speakers.
"Duke - Do you recommend trying to feed a stereo signal to the swarm? Summing left and right to mono? Only running the right channel to all four subs? Something else? " 

I’d do either stereo or summed mono. Getting into stereo vs mono bass would derail the thread and I’d rather not do that.