giver Isaac, it'll make for a good read
Okay.
First off, a watt is a watt. Anyone who knows anything about electricity knows that. One volt at one amp, one coulomb per second, and all that. Yet, it persists that "tube watts" are louder than "solid state watts". Why?
It has to do with distortion characteristics and compression. Transistors are pretty linear devices. Any properly designed transistor amp operating within its envelope will put out a very close approximation to the input signal. And when they're not properly designed, they tend to sound bad. A properly designed tube amp is pretty linear as well. But near the edges of the envelope, different things happen for each amp.
When any amp reaches its limits, it starts to clip. Harmonic distortion increases. It's inaudible at first, up to at least 1% by most accounts. Transistor amps tend to do what is called hard clipping, cutting off the top and bottom of the signal as if you sliced it with a razor blade. This results in all odd-order harmonics: third, fifth and so on. A lot of those don't blend well with other frequencies. The third harmonic is an octave and a fifth above the fundamental, which isn't usually bad, but the fifth is two octaves and a third, and that third will always be out of tune with the same note being played elsewhere. It gets worse after that. Bottom line is that odd order harmonics don't sound all that good.
Tube amps are less linear at their limits. Instead of sharply clipping off the top and bottom of the waveform, they trend to round it off, and do so asymmetrically. That is, the rounding tends to be different on top and on the bottom. The result of this is still harmonic distortion, but along with the odd order harmonics there are a lot of even order harmonics as well. Even order harmonics sound better. In fact, most people think adding a little second order distortion (one octave higher) makes music sound better than it does without it. Fourth order is two octaves. It's nore noticeable, but still nowhere near as jarring as fifth, or even third.
So, what does all this have to do with "tube watts"? Simple. At low levels most amps sound pretty good. The output signal is clean, and pretty well represents the input signal. As the signal reaches the edge of the operating envelope, solid state amps start to sound, well, distorted, and usually not in a good way. So, if the amp is a 100 watt amp, you're going to get 100 watts out of it, and not much more. The distortion on the peaks will keep you from turning it up much higher. And the average output will, of course, be significantly lower than that.
The tube amp, however, still sounds fine. You keep turning it up, and instead of sounding distorted, it sounds harmonically richer, and fuller. It's those even order harmonics. The odd order harmonics are still there, but they're mixed in with the evens, and aren't as obvious. Also, 1% distortion on the SS amp is almost all odds, but 1% on the tube amp is a mix of odds and evens. By the time you reach the point where the distortion starts to become objectionable, the gain is much higher. The average output will be noticeably higher than with the SS amp, even though the amp is still only capable of 100 clean watts, same as the SS amp.
So you can play the same wattage tube amp noticeably louder than you can the SS amp, and have it sound good, maybe even better. How often have you heard that a certain amp has to be turned up to really make it sing? It's the harmonic distortion added to the signal, amking it sound richer and fuller.
Of course, a lot of this is made moot by the push-pull architecture, which cancels out the even order harmonics in the output stage. That's why a lot of guitarists like single-ended amps. More even order harmonics.
