Dr. Floyd Toole, a leading audio scientist and researcher, writes in part 3 of
his book regarding their heat:
"
Solid-state amplifiers typically have output impedances in the range 0.01 to 0.04 ohms (damping factors from 800 to 200).
Tube amplifiers typically range from 0.7 to 3.3 ohms (damping factors from 11 to 2.4),
and occasionally even more. These are large losses, and when placed in series with the frequency-dependent impedances of loudspeakers, they cause audible changes in spectrum; the loudspeakers have essentially been “revoiced” by the tube power amplifier. See Figures 16.1 and 16.2, pp. 434 and 436.
Stereophile magazine measurements of power amplifiers include frequency responses when driving a simulated loudspeaker load. The deviations are sometimes substantial. Somehow this is not discussed in the subjective reviews, when what they are hearing is through loudspeakers that have been modified in ways they do not know. They are not the products the manufacturers designed.
As a general statement, it is advisable to leave tube/valve amplifiers to those who believe that their sound is as satisfyingly warm as the air above them."
