"tube watts" versus "solid state watts"

I agree 1000% with Almarg; I'm no engineer but I'm quite sure that the main difference is that tube amps clip more softly and gradually than solid state. It's not so much a question of what different amps do when they're playing within their power range, but rather how they sound when pushed beyond it.

Beyond the fact that tube amps clip softer than solid state: There's the fact that their distortion when driven hard is composed more of even order harmonics, than SS's predominiately odd order(much more disturbing to our senses). Then there's the fact that(generally speaking) the filter caps found in tubed amps store more Joules than SS amps, and can supply higher instantaneous energy when transients require it. Part of that relates to the much higher voltages at which tubed circuits function. Then(in some amps) there's capacitor construction/speed to be considered. Some info: (http://www.wyetechlabs.com/links/articles/energystorage.html)

Arahl...Some SS amps have circuitry which detects when clipping is imminent, and limit gain so as to behave very much like a tube amp when overdriven. Also, consider that SS amps are, for any price point, much more powerful than tube amps, so that when the tube amp is clipping the SS amp is not.

A Watt is a Watt is a Watt.
An inch is an inch is an inch.
Despite logic and reason, measurements seem to be gathered by perception.
Tubes are men.
Transistors are women..

Unsound...How about a Hybrid?