Why some(tube) amplifiers sound "warm"

This is an extract from Ralph Karsten's comments on 6moons.com review (September 2004) on the Atma-Sphere MA-1 MkII.3 Silver Edition 140-watt monoblock OTL amplifiers. Highlights are mine.

"Warmth. (I just have some comments if that's okay.) Tube 'warmth' (no pun intended) is actually a subjective measure of the amp's even-ordered harmonic distortion, which the MA-1 has less of then the BAT VK-75. So it correctly would not sound as warm, but it should sound more transparent. For the same reason, with less distortion, lower-level details are more easily revealed.

As audiophiles we describe this even-order harmonic distortion several ways, depending on the amount of it: Warmth, bloom, excessive bloom, fatness and finally muddiness.

I certainly understand 'liking' warmth. In the 1960s, General Electric conducted tests and discovered that people will tolerate even-order harmonic content up to 40%! while at the same time not tolerating even a fraction of a percent of odd-order content."