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

@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.

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.

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...

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.

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?