Fred, all these details are described on the Yamaha site, including those for the
663 . Among other things, the 663 is a few ounces heavier than the 661, but this is a trivial matter. It also has a power rating 5 watts higher, but this is similarly trivial. There wouldn't be any apparent reason to suspect that the 663 wouldn't be at least as capable as the 661(i.e., as measured in the S&V lab test)and probably a slight bit better.
As was said before, any amplifier can drive 4ohm speakers and almost any amplifier can be forced to shut down if driven hard enough and long enough with either 4 or 8 ohm speakers. The M80s are a relatively easy 4ohm load and, for example, at any given sound level require less power and about the same current as the M3s, and therefore are a bit easier on the amplifier than the M3s are.
