A bit of that might come from wiring the M5's in parallel and so presenting a lower impedence to the amp... but I don't remember offhand what a single M5 is rated at impedence-wise.

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