This is another of those headphones that specifies maximum input, which in this case is 300mW. If you pump that much in, then the theoretical level of the sound is a ridiculous and harmful 132dB.
Running at those levels for lengthy periods would be very bad for you, of course. But it does also mean that all musically satisfying levels are easily achievable, and that tracks encoded at a too-quiet level can generally be brought up to enjoyable levels.
When we did unleash (briefly) the volume control on a few tracks, they remained clean and controlled in their reproduction, with no undue stresses (except to our ears).
The tonal balance of the headphones was first class. There was no sibilance whatsoever on Nick Cave's rendition of Dig, Lazarus, Dig!!!, but neither did his voice sound muted in the higher frequencies. It was just a smooth reproduction. The bass on Switched on Bach, and on any old track from Primus, was solid and seemed to go down as far as my hearing would. I became curious and ran a sine wave sweep from 5 hertz and up, and it was clear that the headphones were starting to work between 10 and 15 hertz, delivering everything properly from well under 20 hertz.
Dynamically, it wasn't quite up there with the best open designs, compressing the peaks of loud percussive peaks just a little. But as far as closed designs go, it was about as good as they get.
The Yamaha HPH-Pro300 headphones deliver very good sound for virtually all genres of music at plenty high levels and with convenient iPod control. We just wish that they didn't squeeze our head quite so tightly.