Hi krikan,
The QS8s are so placement forgiving that you can wall-mount them darn near anywhere and they'll still deliver a hugely enveloping surround field.
The do not need to be symmetrical in placement. By that, I mean they can be at different heights and distances, nor do they need to be opposite each other. My own QS8s are at different heights, and the right side surround is slightly forward of my couch and about 2 feet higher up than the left one. Their multi-directional dispersion simply blends together with all the reflections from a room's surfaces, yielding surround envelopment that is not localizable at the speakers.
Like the others here, I'd advise against using M50s because of the latter's different timbral (tonal) balance to your M60s.
Regards,
Alan
