IME,the issues with panels and subs is real, but unrelated to sub "speed". I'd guess that interaction between the dipole radiation of the main panels and the omni pattern of the sub is more complex than that created by conventional main speakers and subs. The result is more "peaks and valleys" in the combined response.
If you can get smooth response through the x-over region, you'll be okay. It can be difficult to acheive this, but I managed to do so in my room - after a ton of tweaking my SMS controller. The odds of success without room analysis and PEq strike me as low, since it took a lot of effort even with these tools at hand.
Since my pair of Velo SPLRs sounded pretty crappy with the Maggies until I got the integration nailed down - and very, very good thereafter, I tend to believe the choice of any particular sub is probably much less important than integrating that sub properly.
Marty
PS You might want to scan some of the "subwoofer" threads out there on this forum. There's a lot of good info available on different models.
