Mark (MarkSJohnson) did something nice with his QS surrounds. He mounted the QS8 brackets to some slightly larger wood plaques and then mounted THOSE to the wall. I don't remember if this just gave him better leverage or let him reach a stud that was otherwise too far from the speaker, but it looked real nice and something like that might let you get a more secure mounting.

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