tube watts vs transistor watts?

I agree with Tvad, except some manufactures recommend power needs assuming that the 8 Ohm rated power of the amplifier will "double down" into the lower impedance that their speakers actually work in. That might require more (8 Ohm) tube Watts than ss Watts.

This is good question. Until 2 weeks ago I would have said the same as Tvad and Grinnell. But it may be more than that. I just traded my Bryston B100 rated at 100w 8ohm and 180w 4ohm for an Octave V70se rated at 70w 4ohms (can't find an 8ohm rating). Anyhow the Octave just crushes the Bryston in power and control. Not bashing Bryston for I had mine for 4 years and thought it couldn't be beat without spending a truck load of money. The demo in my home took about a whole 5 seconds to want the Octave (and that was with cold tubes). I think the caveat here is my speakers. They are Dynaudio C1's which are known to like current not necessarily watts. Rated at 85db 4ohms. Logically it makes no sense unless the power is calculated as E X I instead of I sq X R and Octave uses a lower voltage (E).

I hope Almarg chimes in on this one for he always has good technical answer.

So I'm guessing it has more to do with the speakers where tube watts may seem like more compared to solid state. I don't think that applies to all speakers, but some.

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.

...some manufactures recommend power needs assuming that the 8 Ohm rated power of the amplifier will "double down" into the lower impedance that their speakers actually work in. That might require more (8 Ohm) tube Watts than ss Watts.
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IMO, if a speaker requires an amp that doubles power as impedance is halved (meaning the speaker's low impedance spec is appreciably lower that the speaker's nominal rating), then that particular speaker should not be matched with a tube amp.

For optimal results, match tube amps, regardless of their topology, to speakers with flat impedance curves, e.g an 8 ohm nominal speaker with a low impedance spec preferably of not less than 6 ohms.

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