Zest, I took that advice, and bought a pair of M2s for the back surround position, use QS8s for side surround. I listen to music in 7.2 and movies in equal measure. As to music, most of the time CDs, but sometimes SACD or DVD Audio. I have my M2s up for sale. If I had it to do over again, I'd get QS8s for both the back and side surround positions.
My front surrounds used to be a pair of M60 like Michaura M665s, but I moved these gorgeous towers to my office and picked up a pair of Thiel CS 3.6s for fronts, a heavily modified Merak (Axiom made) center speaker with a pair of Axiom 6.5" drivers, a little surgery and a Reference 3a de capo mm tweeter and cross over (the Axiom drivers run free). This is the best sounding center speaker I've heard, although I will be building a slightly larger Axiom shaped center cabinet for a second center using a pair of Axiom 6.5" drivers, my remaining de capo tweeter and x-over, and a supertweeter.
Anyway, sorry I rambled off there ... my recommendation, based on actual experience (at least with the recommended QS8/M2 combo) as opposed to some theoretical model, go with QS8s for side and back surround duty.
Enjoy your system!!
