Are tubes really “warm”?

Tubes and analog seem to be more ambient. This is euphoric and not necessarily accurate, a very old argument.

This is all about that first watt, where low level detail resides. Its not about euphonic character, just more accurate- and since there is more low level detail, more ambience as well.

I always thought the warm sound associated with tubes came from distortion and HF roll off.

The 2nd and 3rd harmonics are responsible for 'warmth', but tubes can have plenty of bandwidth- the old Harmon Kardon Citation 2 had bandwidth to 100KHz and that amp hails from 1959. So the HF rolloff thing is a bit of a myth.

Flipping the coin over for a bit, it is also distortion that causes transistor amps to have brightness and harshness. This is the case even though the distortion in question is quite 'low'. But the ear uses the higher ordered harmonics that cause brightness to sense sound pressure, and so is arguably more sensitive to them than anything else! So even though transistor amps look like they have low distortion on paper, the ear itself doesn't think so and can hear that distortion easily.