I have M3s for my side and rear surrounds now (was running QS8s for about 10 years). My initial thought was to just go with QS8s for the side surrounds and then add something direct radiating like a pair of M2 or M3s to the rear. For other reasons I swapped them all out. I can now really hear the 2 rear channels on 7.1 mixed discs. Before the sound field was smooth and even around the room, but I have 2 rows of seats, and without blasting the rear row by turning up the rear QS8s, I would almost completely lose them. Maybe that was actually a good thing, but I really like having the M3s back there now due to the distance between my front row seat and them.
I went with on-walls and while they are taller than the QS8s, they don't stick out as much.
If you only have 1 row of seats, and the rear surrounds will be a similar distance away from you as your side surrounds, then another pair of QS8s for the rear might be a great fit too.
