Different subwoofer at each corner?

With more than one subwoofer, proper phasing is important. I mean, there is a potential for the two subs to cancel each other out if the phasing is not right. (Apologies for being pedantic if you already know all this.)

For example, one sub cone could be moving forward during a vibration while the other is moving backward, and the result could be that relatively little bass is actually put into the room.

The cancellation is a function of the input music signals (if they are not identical, like a right versus a left), distance between the subs, and the phase settings on the subs. Dynaudio subs have phase adjustment controls, so after you place the subs where you want, be sure to adjust their phases to your liking.

Whatever issues you may be having can probably be tackled by inserting a the fully discrete two channel SVS Sub-EQ unit into your system. Especially beneficial if the trail and error process of moving your subs into different locations in the room and trying passive treatement is not your cup of tea.

Actually, using 2 different subs (designs, brands, etc.) can be advantageous, as they'll each load the room differently.

I agree that 100 Hz is probably a higher crossover than desirable. I always start at 0.7 x -3 dB point of the speakers. That may seem low (particularly when starting with a 100 Hz crossover, but you don't want to hear the subs. You want to hear a seamless full-range speaker. The low of a crossover point will help blend the subs with your speakers.

Don't be afraid to try the speakers *near* the corners. I was anti-corners until I moved into room where I couldn't get other placements to work. Per the advice of Roy Johnson @ Green Mountain Audio, I went against my "truism" of sub placement for 2-channel audio. I'm glad I did.

Phase-based cancellation isn't necessarily a bad thing - if you use that cancellation intentionally and strategically. In a previous room, I used one sub to remove a node accentuated by my primary speakers, and another sub to fill in the low end.

And now for my most controversial statement...

Seamless sub / main speaker is the typical goal. Don't get me wrong, it's a worth goal. However, you'll know when you have ideal physical / phase placement when the sub performance enhances the imaging of your primary speakers. Crazy, eh? Whenever I read that someone "moved their speakers 1/2" and the imaging snapped into place", I believe that has nothing to do with the tweeter / midrange placement, and has everything to do with how the room is being loaded with low frequency information *synergistically* with mid-range information. You'll know it when you hear it.

Nrenter: Can you help us mortals with the math, please?
.7 x -3db means what? Although I know that my speakers are 92db, are you suggesting .7 x 89db = 62.3 as my crossover point? Where did you learn this one? Just curious.

If I understand Nrenter correctly, this means 0.7 multiplied by the frequency at which your speaker rolls off -3db in the bass region. A common speaker measurement, and what everyone is generally referring to when discussing how low in the bass a speaker can go. So for example, my speaker's -3dB point is 70Hz, meaning that according to Nrenter's rule of thumb, I should crossover at about 50Hz. Which interestingly is where I have the crossover set. I hadn't heard this rule of thumb, but using an SPL meter to calibrate the subs and mains, 50Hz was the point where I could get the flattest bass response. Above that and I was getting some humps between 80-100Hz.