I was thinking the same thing as Fred, just dump Audyssey and try it with a 80hz XO for the rears, sides and center and 60hz for the mains and see how it sounds.
I am thinking the M2s wouldn't be any better than the QS's, they have a high roll off point as well according to their graph. Maybe a set of M3's with that upper bass hump/depressed mid graph would be the better choice for rear duty in that large of a room, other than a second sub placed behind the listener to even out the bass response through out the room which really appears to be the main issue.