I think most people prefer two M2's in the back for 7.1. I however, only have a 6 channel amp and my back wall is only 12', so I bought one QS8 for the rear and it works great!
Also, any movie in 5.1 can be changed by the reciever into 6.1. Some movies really benefit from this. One that I can think of off the top of my head is Independece Day. There are many back surround effects that seem like they were originally intended for 6.1, but they were not. Also, many movies are actually 6.1 mixes, but the packaging does not say it, i.e. Atlantis, or Jurassic Park 3.
The Hulk was 6.1 theatrically, but the DVD does not say it on the packaging, but it still is a 6.1 mix.
