Daniel,

Some current and vintage models of Adcom power amps (such as the GFA-5002 and GFA-5006) are designed to be "bridgeable" -- you can combine two channels in 180-degrees out-of-phase and drive one speaker by these two channels in a bridge-circuit configuration. But unfortunately, your GFA-7000 is NOT designed bridgeable.

Anyway, although your room is fairly large, 200W/ch would be plenty given the high sensitivity (efficiency) of the M80s.