I have a single QS8 in the rear, 2 as side surrounds and they work great, no tonal differences when listening to music in 6 channel to match my M22's and VP 100. They all seem to play equally loud as well.

Just wondering why you say the M2's have something to be desired or is just upgradeitits? Remember in 7.1 the rear channels really don't receive much information to use during movies, at least not as much as the sides, so you may not notice the rears as much.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70