This is just more proof that not all Yamaha's are capable of driving the M80's which is why Axiom does not recommend Yamaha products for use with the M80s. But yes, I am also a little surprised.
Let me try to explain a little better.
The Yamaha can drive the M80's. It's the way that the Yamaha drives the speakers that's in question. As another post in another forum put it, the receiver he was using seemed to blend all the instruments together creating a muddled effect. Using the amp clearly defines and separates the instruments and vocals. In addition when the Yamaha was cranked up the sound becomes increasingly flat and distorts a little bit. The receiver was very hot as well. When I ran the XPA-5 at higher volumes the sound was consistent at all levels.
I am by no means an expert in any of this, but this qoute from another forum seems to make sense to me:"However, the advantage that the separates enjoy comes from everything being, well, separate. By separating the power amp from the pre-amp, you get less noise, better bass, and a more dynamic sound - punchier is the term that is generally used. The pre-amp is able to work better because it doesn't have to share the power supply and chassis, etc. with the noise producing transformers and such that are found in power amps.
This also gives the designers more room to work with, the circuits can be spread out a bit more, thus helping to reduce noise even further."
The above quote seems consistent to what I experienced.