tube watts vs transistor watts?

Just because a speaker says "200 watts power handling" doesn't mean that it requires 200 watts to function. The 200 is a maximum, not a minimum and only a very loose maximum at that. Ratings in watts tell you very little about how any amp, tube or transistor , will drive any particular speaker load. Many lesser rated amps of either type will drive speakers that amps of higher rating have trouble with. The output transformer of tube amps smooths out difficult loads in many cases. Conversely the early Classe SS amps were rated at about 25 watts but would drive one ohm loads that would blow up many amps of 200 watts or more. The moral, all watts are not created equal, tube or transistor.

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.