With 12' of space behind your seating area, I'd personally go with direct radiating speakers. Many of the 7.1 sound tracks out nowadays have discrete R/L SB channels with things like gun shots, walking behind you, breathing, growling, etc. The sounds from these rear discrete channels tend to be sounds better suited lower than the side surround too. I'd put them at ear level or just slightly higher if you can. The Q's work great for the RS, but I can't help but believe a direct radiating speaker is the way to go for the rears if you have the distance, which you have plenty. I'd actually try to talk you into M22's or M60's if you have the funds. I have tried M3's and found them to blend nicely, as the rears do not have to match the tonal frequencies as much as the front R/L and center. It my case though, I only had about four feet behind me and QS8's work much better than the M3's. I envy your space. Your options are wide open to put just about anything in that room you want.
