I bought 4 subwoofers and I'm absolutely not doing a DBA! Hah!

@erik_squires, I should have specified that I was thinking of the woofers being side-firing (and therefore back-to-back), and now I understand that the woofer motors are too deep for that. 

I really like your idea of using the subwoofers as speaker stands, that’s a very efficient and elegant use of available floor space.  Ime in this sort of setup the imaging of the mains is improved by the use of spikes underneath the subs.  Even if you can only use one spike (like front-and-center), imo it makes a worthwhile improvement.

The only other (weird) configuration I can think of that might be worth considering is one woofer down-firing and the other front-firing.  Again you’d probably have to shift the plate amp northwards. 

Duke

Does the plate amp have to be in the cabinet?

Well , if it's not in the cabinet it's usually just called an amp 🙄 

@erik_squires 

How are you going to isolate the speakers from the subwoofers?  Are you concerned that the bass vibrations will affect the overall sound?

@erik_squires Here's something to think about. The reason a DBA works is by breaking up standing waves. But if you don't want to have multiple subs for that, one way you could handle that is by having one woofer aimed at a nearby wall at 45 degrees, so its output bounces around the room like a cue ball.

I did a show recently where I did exactly that. So the audience was looking at the corner of the sub rather than having it face them directly. This really helped with getting full bass at the ideal listening positions.

Might not make for the most attractive cabinet though...