I'll take a stab at this since I am in a similar situation with my counch along the back wall and rear mounting the speakers. The general consensus around here is to have the surrounds at a height about 6 feet off of the ground (3 feet above listener's ears). However, you don't want them right up against the ceiling either, but with 9 foot ceilings, you shouldn't need to worry about that. As for width apart, I have heard 2 different models of thought. One is to have them in line with your front speakers, and the other is (if you are sitting on the back wall, and have your QS-8's rear wall mounted) to put them just a little bit wider to create a larger sound field.
Most of this advice came right from Alan when I asked the same question. Just remember that the QS-8s are great speakers and are very forgiving. Of course, if you can move your seating a couple of feet away from the back wall, that would be best, but I can't do that myself.
