Tube Watts vs. Solid State Watts - Any credence?

well some of them isn't most of them. it's nature of tubes and output transformers. they're weak when impedance of speaker goes down on low frequencies.

Some actually think that only tubes can make the bass sound right. Or at the vey least something like Lamm hybrids.
This is probably a slight exaggeration and a matter of taste and speaker choice, but I can see a strong point.

Tim deParavicini (EAR) has said that he can tell what an amp's bass will sound like by looking at the power transformer. For bass, the larger the better. Unfortunately, the larger it is, the worse it is (all else being equal) at high frequencies. An amp designer has to balance the two against each other and find the best compromise.

Yes, a watt is a watt, but, the way most tube manufacturers measure their gear, a tube watt is, arguably, LESS than a solid state watt because tube manufacturers typically spec the output at a much higher allowed distortion level.

BUT, measurements notwithstanding, I actually have heard many demonstrations where a low-powered tube amp is perceived to sound louder than a much higher powered solid state amp. In such demonstrations, neither amp was pushed to obvious clipping. At modest or low sound levels, I think that many solid state amps sound dead and lifeless. To get them to sound livelier, one tends to push the volume. When heard side by side at the same volume, one is often struck by how much better a well designed tube amp sounds compared to a comparable solid state amp, and because so many audiophiles conflate sound characteristics down to some simple measurement, like power and distortion, they concluded that the better sounding amp is more powerful.

well some of them isn't most of them. it's nature of tubes and output transformers. they're weak when impedance of speaker goes down on low frequencies.