tube watts vs transistor watts?

Depends if you're north or south of the equator.

Tvad, it's possible for a given speaker to have both a low nominal and minimum impedance.

xti16 - as to your reference to power calculations, it makes no difference which form of the equation you use, they are synonomous. To state otherwise ignores ohms law.

TVAD, it is :-). My speakers are spec-ed at: 4 Ohms nominal, 4 Ohms minimum. Many, if not most tube amps have 4 Ohm taps. In such a case, if one was willing to provide double the 8 Ohm tube Watts as a similarly rated 8 Ohm "double down" ss amp, all should be equal. If the speaker impedance load was 16 Ohms, the opposite would be true. Of course, I'm sure you knew that, but, it does clarify things for the OP.