Technically speaking, the perceived "warmth" in the sound of many tube amps (besides the psychological effect of the beautiful heaters
) comes from their high output impedance (low damping factor). When amp's output impedance is high, the overall frequency response is biased toward the speaker's impedance curve. Most speakers exhibit large peak(s) in impedance curve at bass frequencies. So, when driven by a high-output impedance amp, these bass frequencies are enhanced, resulting in the perceived "warmth."
