You should check with an Avia disk or equivalent to make sure that the monoblocks didn't invert the signal compared to the amp in the receiver (equivalent of having the speaker wired backwards). If you do find that main and centers are out of phase, just reverse the speaker wires on each of the M80s.

I can't think of any other potential problems -- everyone who has added amplification to their M80s seems to have been very happy with the results. Those puppies seem to like power.

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