To reiterate what Alan said, I have a M60ti/VP150/QS8 5.1 config, and have experimented with various QS8 positions. Using mostly multichannel SACD and DTS/DD 5.1 music, roughly 90-100 degrees sounds the best to me. When I put the QS8s back at 135-140 degrees (on back wall) it sounded OK but there was loss of side envelopment.
Likewise with Dolby PLII-synthesized surround from stereo sources, about 90 deg sounded best.
It's interesting that Tom Holman of THX strongly feels that 90 deg is best for side surrounds: http://tinyurl.com/2fh33
