^^ Actually Mapman this is not entirely true.
Because tubes usually are more linear you can often build lower distortion circuit with them- and often without feedback. That is a lot harder to do with semiconductors!
Most power transistors incorporate a non-linear capacitive aspect into the junction of the device that is part of the price of building the junction in the device. The capacitive aspect is magnified by the amount of current put through the device, and in case some don't know it, current is a big deal with semiconductors. This capacitive aspect contributes to higher ordered harmonic distortion and is part of the semiconductor signature that is very hard to eliminate! Some semiconductors take advantage of this capacitance- for example most modern FM radios are tuned by the use of a varactor diode, which changes capacitance as the voltage applied to it changes.
Tubes have a similar capacitance but it is far less significant, lacking the non-linear aspect and mostly is an interaction between the input capacitance and the source driving the tube and is known as Miller Effect; IOW can affect bandwidth if not managed.
Further, distortion at clipping can play a much bigger role in the sound of many stereos than you suggest. Musical transients are where the action is- and if for whatever reason the amp is forced into clipping for an instant it can have audible effects. There are a number of threads on this forum asking about the difference between tube and transistor power as for some reason tube power seems to be more profound than transistor power.
The reason for this is how the two distort- when a transistor amp clips its instantaneously audible- when a tube amps clips it may not be audible until the amp is clipping fairly hard. Instead, it will make more higher ordered harmonics that interacts with the human ear/brain system in a way that will make the sound seem to be louder than it really is.
In a nutshell this makes the tube amp seem more powerful than a transistor amp of the same power as the outright clipping of the transistor amp seems to describe a limit that the tube amp does not seem to have. Its important to understand that this has a lot to do with how we perceive loudness and that the tubes play into that in a way that transistors don't.
The difference of course is revealed as soon as you put a sound pressure meter in the room- then it will be seen that both amps are playing at the same volume.
Anyway, the result of this is that the common wisdom is that if using a transistor amp you need a lot more power to avoid audible clipping.
But I agree, avoiding clipping altogether is preferable :)
