Hi Rizzo,
Just a couple of answers to questions you posed. Re the QS8s and what height to mount them: per Dolby Lab recommendations and my own experiments, I suggest anywhere from two feet to six feet (a couple of meters) above your ear level when seated. It is NOT critical; the QS8s don't have to be at the same height and may even be positioned asymmetrically, as my own are. Just don't put them too close to a ceiling. If the room has a cathedral ceiling, then put the QS8s higher up if you want. You can experiment with height by using a couple of stepladders to temporarily position them before you drill any holes.
In passing, 12-gauge speaker cable is more than adequate for runs up to 60 feet or more. I call 7.1 vs 5.1 a "modest enhancement," depending on the room size. For big rooms, using four QS8s will certainly give you better coverage and envelopment of the surround field over all the seating areas.
Regards,
Alan
