Although I must claim total ignorance, as I didn't past the comment that tubes are warm because they produce harmonic distortion.
@pedroeb To be clear, all amplifiers produce harmonic distortion. In general there are two kinds of harmonic distortion- the lower orders (2nd and 3rd) and the higher orders (5th and above). The lower orders are well-known to be innocuous to the human ear. The higher orders are well-known to be unpleasant if audible. This fact has been known to the audio world for some 90 years or so. If you want a reference see the Radiotron Designer's Handbook, 3rd edition, which was published in the mid to late 1930s. I think its available on CDROM.
The 2nd and 3rd harmonics contribute to what audiophiles call 'warmth'. If not masked, the higher ordered harmonics can cause an amplifier to sound harsh and bright, clinical, sterile and other such audiophile terms.