Just to get this straight, you're not talking about combining the two amp sections, are you? Because that would probably result in a puff of smoke, and perhaps a fire. I don't think putting a high power output into a low power (line level) input is a good idea.
That said, also remember that double the power is not anywhere near double the volume. (see Alan's article in the Late Summer Axiom Audiofile).
How big a space are you trying to fill? And how loud do you need them to be?
Probably a good way to go is to use the c270 to power the M80s using the c370 as a preamp.
