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.
That's an interesting idea using two direct firing, maybe a pair of M2's or M3s. What about using a VP150 or would it over power the QS8 surrounds?
