Also if you have your crossover points on the VP180 and M80's set below 80hz or to the LARGE setting, try them at 80hz each to see if that will make any difference. Dips at and below 4 ohms and at frequencies below 100hz is the most current demanding for an amplifier. Especially in instances where the speakers are being played at reference volumes which you are doing.

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne