This caught my attention - if you had to guess, what particular frequency/range was causing the rumble? This is EXACTLY the type of $h!7 I'm trying to get rid of...
I can't comment on what frequency range was giving nostatic trouble, but when I got my first MAS cab, I deliberately put it into spots on stages where I'd had trouble before with multiple cabs and the difference was marked. One corner, in particular, where I'd gotten huge boom with the house 410, my trusty 2x12", and my little Markbass combo, I had no problems with my 66 (the first MAS cab I got). I even tried shifting it around to get that boom and couldn't. Didn't do anything special with the amp. Tried again the next time out there to be sure, with the same result. Similar results when listening to someone else sitting in and walking around the room -- bass is much more even in the crowd. More even, in some cases, than everyone else's instruments in the PA. Definitely more even than the sound coming from their amps on stage. Instead of the bass dropping out in one part of the room and being overwhelming in another, you notice that it's just a little quieter here, a little louder there. Not a magic pill, but a huge improvement nevertheless. Works well with and without PA support.
That's not to say that the MAS cabs are immune to room issues -- too much cab, turned up too high, in too small a room can overload the room maybe a little more easily than a conventional cab can, and a school gym will still sound like a school gym -- but they do seem better able to deal with hot/cold spots and less prone to booming on specific frequencies. My 46 needs to be elevated, otherwise there's too much bass clouding the mids (I probably should've elevated the 66, too, when I had it, but I didn't think to do that) but, all in all, I've not worried about where I put my cab (not for sonic reasons) since I started playing the MAS stuff.
EDIT: The cabs kill outdoors, by the way.