My biggest concern with using a tube amp with the M80's would be the issue of clipping.
From what I understand of the M80's design and their power handling capabilities, to get the most out of them especially in large rooms is to power them with a robust solid state amplifier.
Now as I type this I am listening to headphones powered by a tube amplifier. The sound is quite nice actually. However, I cannot attribute a "warmth" to the music in comparison to any other source/amp I have used with the multiple headphones I have. On the other hand it is very soothing listening to music in complete darkness and watching the tubes glow.
For the majority of tube amps on the market IMO they would be better paired with the Axiom bookshelf speakers (M2, M3, M22) in a small to medium sized room.
