I would second Mark's question. With QS8s on the side, they are going to reflect off that back wall - in about 10 seconds! The point is, you will get a surround image without sound directly from that back wall. Try it without the hassle of the 7.1 setup and see how it sounds. You can always add more speakers, but your set up might require better processing and delay capabilities than the average AVR has to offer. Not sure what you have going on in terms of equp, but in theory you may need a 9.1 setup to cover the gap beteen side and rear. (Ok, now i'm just making numbers up.)

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire