Assuming that both measurements are made in the same manner and at a low THD limit, say 0.05%, - then the answer to the last part of your question is "100wpc". The biggest reason why tube amps with less power are acceptable is that the distortion in a tube amp is both more gradual and of different harmonic content than the distortion in a solid state amp. The tube sound is mainly from these distortion properties (and I expect from the output transformer where one is used). One would be hard pressed to tell a difference between a solid state amp and a tube amp amplifying a signal at a setting where both have the same thd.
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