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.
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.