Crossing as low as possible probably what's making it work. However, technically, using the miniDSP 2X4 and going L/R in to L speaker/L subwoofer and R speaker/R subwoofer out means that you're using the miniDSP as another crossover. You're taking the L signal and R signal crossing them over separately from each other, which means you now have stereo subs aka it's still technically a 2.0 system albeit a 3 way system instead of a 2.2 system. Now this can cause a lot of problems if you cross too high, but can also yield some pretty good results if you know what you're doing and what to look for when matching the sub to the woofer. Now the difference, which I'm sure you've caught on to by now, is that when you cross over using bass management it crosses both the L and R signals to the sub (or more in a surround setup). This a mono configuration and lends itself to crossing low so the sound is omnidirectional and allows you to place subs all over the place. If you cross high enough with stereo subs then the subwoofer has to be placed very specifically in line with the woofer (although now more midwoofer) and tweeter and height could even be important as well.
I'm pretty sure that's all mostly correct, but corrections are always welcome.
I was always a "tower" guy, then I added a sub, it was a 15" ported sub, decent enough reviews, nice sound, but it just was not rite, unnatural, seemed forced.. So I played with crossing, found the lower you cross the better the sound and integration, now that doesn't mean cross at 40hz, I found between 58 and 82hz is low enough for most systems and rooms... But the problem was the ported subs bass is all wrong for the music I listen to, now if I put on some Dr. Dre the bass sounded fine, not sure why that is, but with the acoustic stuff the ported sub sounded, well, BAD...
So I bought my first sealed sub, 10" and there was a big difference, the bass is faster, smoother, and integrated much better with the mains... Then I bought another one and it was all over, dual subs fixed a lot of the room issues, with a single sub you could place it with duals you swear the bass is coming from the mains...
I have since played with this a lot, I found that if you setup the system rite, paying close attention to balance keep the subs with the mains {inside seems to be better but that may because it allows the mains to be further apart}, cross as low as possible, and use a nice sealed sub the results are better than any tower you can buy form much less money...
My little csb1's with the svs12's sound HUGE, people that don't know the 2 boxes on the floors are subs hear it and say "wow them little speakers make all that bass", I explain to the the subs are 12" and 350watts but they still don't get it because it sounds like everything is coming from the bookshelfs....
I think I understand what you are saying about the 3 way vs 2.2, but I split the signal from the mini dsp and send the same signal to both subs, so you come out of the mini dsp go to your first sub with a pair of rca's and then another pair out of sub #1's line out to sub #2's input...
And when I balance them I put my meter at the listening position I turn the subs off and set my preamp to say a certain 80db, then I shut my mains and play a 45hz test tone through one sub at a time to 78db each, then I play both subs and see what the meter says if its way over the mains level I will turn each down to say 77 and retest them together... Then if I want a little more bass I can always use the parasounds boost to get it, because I hate to readjust once I get it perfect....
