But what is the point of adding a token valve to something like a DAC? It's just a sales gimmick. Transistor small signal amplifier distortion is so low as to be negligable. It certainly can't be heard in a quality design.
Amplifiers are usually engineered with a flat or neutral response and warmth would imply an upper bass / lower mid hump, so you can't really have a well engineered warm amplifier. Perceived warmth is a speakers / room thing IMO.