Ok folks, so the point is here not that the tube Watts are more "powerful" than solid state Watts in absolute terms (when measured) but they are more powerful in terms of psycho acoustics. It's the subjective finding that not all Watts are the same. So when someone is saying that a 20W tube amp is more powerful than a 100W solid state amplifier you have to correct that somewhat by using abovementioned explanation.
Chris