I would not discount the vibration-cancelling effect of having back-to-back drivers properly wired. Idoing this is DIY speakers and it's very significant. If side subs won't work, you could have one facing forward and one facing backwards, keep the same footprint and get the canceling effect. Pretty significant for subs with monitors sitting on top!
I bought 4 subwoofers and I'm absolutely not doing a DBA! Hah!
I just received shipments of 4 subwoofer drivers and they will not be going into a distributed bass array.
I'm replacing my left and right speaker stands with powering subwoofers with 2 subwoofer drivers each. I call them powering because they will be powering the 2-way monitors that will sit on top. Here's a beauty shot of the insides.
The amp has all the DSP power I could need to produce perfect speaker measurements. I wonder if JA at Stereophile would wax poetically if I priced them high enough?? 