I'm assuming that when you say "the wall that is slightly behind the listening area" you mean mounting them on the back wall of the room. If so, That WOULD be sufficient. It would just be better to mount them on the side walls because of the directions the drivers fire. But mounting them on the wall behind would work.
Unless one has a room specifically dedicated to HT/music, nearly all of us have to make some compromise in speaker placement due to physical restrictions of the room, or furniture, or the WAF/SAF factor. Don't agonize over it. Just give it your best shot. The QS8s are pretty darn forgiving when it comes to placement.
