I think my confusion is you keep using the term "rear". In 5.1 setups you have left/right surrounds, no rear channels. In 6.1 or 7.1, you still have the left/right channels, and then you add one or two rear channels.
If you have your left/right surrounds, which should be to the left/right of you (not on the rear wall), and your adding a Qs8 to go in the middle of them, I don't think your going to gain much by doing this. You would be better off placing a Qs8 to the left/right of you, even if they are on the back walls spread out as far as possible. Then place 1 or 2 m2's for the rear channel signals.
The Qs4's and Qs8's are intended to be used for the left/right surrounds. Since the rear channel does not get much information anyway, unless your using PLIIx, you would be better off using the m2's in that position.
