tube Watts vs solid state Watts

It isn't. Contrary to audiophile myth WATTS ARE WATTS. It makes no difference whether they are tube or SS watts. They all measure the same.

A watt is a watt, an amp is an amp, they all just amplify a signal, and probably sound alike. Or not........

The limiting factor isn't just wattage - the audibility of clipping comes into play as well.

Tube amplifiers "soft-clip" - that is, they do not generate as much harsh-sounding high-order harmonic content when they clip as solid state amps do.

Perceived loudness is more a function of the average SPL than of the peak SPL, and a tube amp's relatively benign behavior when clipped means that it can be driven farther into clipping before the distortion becomes audible and/or objectionable. So the average SPL with a 20 watt tube amp might be 2 or 3 dB higher than with a 20 watt solid state amp by the time clipping becomes audibly objectionable.

Duke
dealer/manufacturer

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.