I think the normal recommendation for surrounds is "beside the listener and up a bit" rather than behind. That might have something to do with what you're hearing...

My QS8s won't be here until Monday or Tuesday so this is just a guess

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8