I've had so many people say "Tube amps are so dirty." because of the 'WARM' hummmmm. It imparts color to the music that is missing with CD's and modern amps.
They also tend to impart even-order harmonic distortion, which is not offensive to the ear and is often pleasing. Odd-order distortion puts our teeth on edge. Not sure of the neuropsych reason for it...
