I think some cooling fans and a higher crossover should help, as this would take quite a load of the Onkyo, how much longer you could run at this level would be unknown. Better yet would be to get an outboard 2 channel amp to driv ethe M80s for this high volume levels. I am looking at the Pro PA amps out there like the Crown XTI1000 or Behringer2500, quite cheap for the power output. There are some reviews of owners running these amps like this and they are all very positive.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70