My room isn't much larger than yours (12.5 X 18). My rears are about 36" behind me. I have tried 5.1 and 7.1 using both M3's and QS-8's as surround backs. The M3's did not blend well at all due to being too close and the fact that they are directional, but the QS-8's work nicely and I prefer them over the 5.1 configuration. I have not heard QS-4's. If the cost delta isn't an issue, go with 4 QS-8's. Many BR's have discrete 7.1 and even though you are close to the rear wall, you will still be able to enjoy the extra discrete channels.
