M60 are great speakers. As for my new 7.1 setup I was not sure if the QS8's in the back will be as good as on the sides but they are just great. The back row is against the wall and they diffuse the sound enough to not be directional even when my ears are less than a meter away.
Interestingly the biggest impact they make is with Circular Surround II mode on my Marantz (their alternative to Logic 7). With QS8's in the back this mode sounds much subtler and is now one of my favorite. It is not for everything but some concerts sound amazing. One CD that I can listen to again and again is
Fado Curvo by Portuguese singer Mariza. Though this is a studio CD, 7.1 setup sounds as intimate as if she is singing for you in one of Lsboa's small taverns. You can swear there are guitarrists sitting in front of you and echos from the arched walls in the back. If you like blues and female voices this is highly recommended.
