How far are the side walls? Lots of people use QS8's for side surrounds, and then use a direct firing speaking such as the M2 or M3 for the back speakers, particularly when the back wall is that far back. You should have no problems as you can adjust the distance and channel level with your AVR but I would recommend direct firing speakers for the rear. Someone who actually has this set-up can probably give you more information.

-David