For the rear channel it depends on the distance to the back wall from your seating position, I would go with a direct radiating design for distances over 6-8ft to give the speaker room for its natural dispersion, any closer than that and I think the diffuse sound of the QS series is the way to go.

I guess alternatively if you were to mount two direct radiating speakers closer together than John's suggestion, you might end up with a sound similar to a single QS8 but have more impressive impact of a direct firing speaker into the listening area.

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