Hi Lex how about I try to create more confusion for you.
I’ve done lots of experimenting with 5.1 (QS8s), 6.1 (QS8 surrounds and either a QS8, M22 or VP150 rear) and 7.1 (QS8 surrounds, M22 or QS8 rears). My experience indicates that what works best depends most on the shape of the room and relative seating positions to the side and rear walls.
In a relatively square room where the side and rear walls are close to equal distance from the seating I’ve found that by placing the 5.1 surround speakers farther back they create a phantom rear image that has even sounded better to me than 6.1 or 7.1 while still maintaining a seamless side surround stage.
In a longer more narrow room where the side walls are closer to the seating than the rear walls 5.1 surround speakers can’t be placed optimally to create both side and rear surround images without there being gaps in the 360 degree soundstage.
6.1 helps fill in the rear soundstage some while still allowing for optimal side surround speaker placement. However, as I believe Alan has pointed it can be difficult to tell if the sound from a single speaker directly behind you is actually coming from behind or from directly in front. In this respect a single QS8 performs a little better than a single M22 or M2 since it spreads out the sound behind you better.
While IMO 6.1 can sound a little better than 5.1 in a narrow room it’s not even close to how much better 7.1 sounds. As for QS8 vs M22, or M2 working better as 7.1 rear speakers I use to think that if there was enough room behind your seats (about 5 or more feet) then direct radiating speakers like the M2 would sound about as good as QS8 speakers. I’ve done a lot more listening since then and changed my mind feeling the even with 8 feet behind me in my present room the QS8 clearly outperform the M22 and M2 speakers in the 7.1 rear duties.
So if money is not an issue I highly recommend you stick with QS8 speakers for both side and rear surround duties.
Cheers,
Dean
