Alex,

Look at this. Your Yammi can push over 140W into a 4Ohm load (like the M80s) with negligible distortion.

For your reference, I listen to music with my cheap Denon and M80s at about a half watt per channel and movies at a few watts. That gives me headroom of more than 21dB which is more than plenty.

If you are interested, you can read more about some experimental data and discussion here.