No matter what anyone says, 40 watt tube approximates 100 watt SS. The size of the room is the dominant factor in this equation.Is there some law of physics I missed in school? This may be sorta true in some cases, with some amps but it's far from a universal conversion factor. I can think of many circumstances with my own gear collection where this is absolutely not true.
For instance, what about damping factor? While I'm not a nut on the subject I will say that a 40W tube amp w/ a DF of 3 will not sound anywhere near the same as a 100W SS amp Class A/AB w/ DF of 1,000. Speaker Q is fixed by the design and the the same speaker will be driven very differently with amps of varying DF. That is not to say that both amps may not sound good (which is the decision of the listener) but they definitely will not sound the same.