Why is a tube guitar amp louder than a solid state guitar amp?

Question

For example: Why would a 30 watt tube amp be louder than a 30 watt solid state amp at maximum volume?

Answer 1

The way the human ear works is that it is very sensitive to the harmonic content of a sound. A tube amp is less linear (that is, has more distortion) at signal levels below clipping than a solid state amplifier. The distortion will increase slowly, and then more rapidly as the amp starts to clip. In fact, the distortion increases so gradually and is of such a benign nature that the onset of audible distortion has no easily defined threshold. The solid state amplifier on the other hand has no such gradualism. It is almost perfectly non-distorting right up to the point that it clips, and then it clips HARD. It's easy to hear the threshold.
This sudden onset of distortion is also composed of relatively harsh sounding distortion, not like the subtle second and third harmonics of the tube amp. The human ear hears the sudden harsh distortion as clipping and harshness. It interprets the low order distortion of the tube amp as a louder sound, not as distortion. In effect, the tube amp fools the ear into thinking that its early distortion is more loudness. They therefore sound louder or more powerful than the actual measurements show are really there.