I think there's going to be little to no advantage to biamp the M80s, especially if you're doing it with the same unit. Bi-amping from separate amps with separate power supplies is questionable enough, but doing it with one amp with the same power supply just doesn't make a whole lot of sense. Real bi-amping comes in when you've got an external crossover before the amps.

I am the Doctor, and THIS... is my SPOON!