I’d go with the Q’s. You have a similar situation to what I’ve got. I’ve got the M3’s in the SB locations and I’m going to swap them out for Q’s as they are just too close.

Whatever you do, definitely go with back speakers though. The 6 and 7 channel sound is way cool. There’s not a lot there, but enough to make it worth while. I’m enjoying the hell out it. Audioslave at around 100 db’s in 7.1 is better than apple pie and ice cream.