tube Watts vs solid state Watts

Duke, excellent answer. Clipping behavior is the reason that people perceive tube amps to be more powerful even when the wattage is the same.

This is also the reason that electric guitar players love tube amps. Many of the most memorable "sounds" we hear from the great guitar players are due to the way they drive various gain stages of their tube amps into intentional distortion. Do the same thing with a transistor amp and it typically just sounds nasty.

Just for full disclosure, I current use one of the solid state class T amps. If you stay out of clipping the difference between solid state and tubes is much more subtle.

While on one level a watt is just a watt, we also have to take into account how humans hear. As Duke noted early in the thread, we perceive "loudness" far more on the average level than peaks.

Take the following example with a 20 watt tube amp and a 20 watt transistor. Both are plugging along into a 87 dB efficient speaker. We're using roughly 10 watts to crank out a loud 96 - 97 dB average volume. We encounter a 3 dB peak in the music which requires doubling the needed power. Both amps deliver their maximum unclipped volume without clipping.

However, if that is a 6 dB peak, we now need 40 watts so both amps clip. The tube amp will give a better behaved clip that is less likely to draw attention to itself so we leave the volume control alone. The transistor amp clips harshly. Many people will respond to that harshness by turning the volume down until the harsh sound goes away.

This is the mechanism that causes many to say tube amps sound more powerful, watt for watt, than a transistor amp. Nothing real complicated going on.

Keep in mind that powerful transistor amps are widely available so if well matched with the speakers, this isn't a big problem these days.

As for transistors delivering more current & power as impedance drops, this only happens at certain frequencies, generally in a reasonably narrow band. You only get the extra power if your peak just happens to be composed primarily of energy at the correct frequencies. Music isn't generally composed and played with much consideration that the loud parts conform to a particular frequency profile. As such, it is a roll of the dice as to whether a transistor's extra power at lower impedances will be a benefit when playing any given piece of music.

All of that said, one is still better off buying an amp of appropriate power for your speakers and listening habits. The better solution is to avoid clipping and the need for impedance-related power all together.