I cannot recommend strongly enough that a pair of M2's or other direct radiators are the best option for rear surrounds in 7.1. To my knowledge, rear surround information is meant for directional speakers, and if you want to get into DVD-Audio or SACD, these high resolution 5.1 formats sound much better, in my experience, with the surround channels directed to my M2's than my QS4's. I have done it both ways, and to my ears the sound of 5.1 music is much better with direct speakers all around. A pair of M2's is less expensive as well. In fact, unless your room is huge, you could get M60's instead of M80's, and have more than enough money left over for the M2's
