I have never tried a QS8 for my rear channel in my 6.1 setup, but I do have a M2i there now and it works very well. I have noticed that very little actual sound comes from the rear position, even when watching Gladiator, Which is the only movie I have that plays in DTS 6.1 discreet mode. I can tell you that the 5-6 DVD-As I have sound great from all speakers.
Just my .02