It depends on how much room you have behind for the rear channels. If there is a significant amount of room for diffusion to take place(min~6ft) then M2s could be used, M2s because they are a better tonal match for the rest of the speakers. If you have less room then I would suggest using the QS series again the nod goes to the QS8s for the same driver compliment thus same timbre match.

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