Z, the QS8s provide plenty of dispersion for general ambience, so I'd suggest that you consider M2s to have a little more direct sound in back; this would be a bit better for movies where some sounds which are about equal in the left and right surrounds are intended to be specifically imaged in back. The M2s should be about 6' up on the back wall and separated by about the same distance as they're behind the sitting position. Oh, and incidentally, set your Yamaha to 8 ohms impedance so that you don't limit the maximum power available.
