SET amps and speaker sensitivity.

In SET amps as with many others, all watts are not created equal. Other aspects than simple output figures determine abilty to drive real speakers. In particular, the size and quality of output transformers is a factor. Now they are heavy and expensive, so in simplistic terms, the more you pay for an amp, the better it will drive speakers for a given output.
AS Ultrakaz says speaker impedence is very important for SET's, the speaker may be quoted at 8ohm impedence, but have nasty dips to 2ohms in the frequency range. A SET won't respond well to that.
The answer, try your speaker and SET together preferably in your own rooml, see how they match.

This, Maria, can be a proverbial can of worms, but here goes...

Regarding speakers, 'high-sensitivity' MIGHT be defined as anything at or above 93dB, but there's NO concensus here. As the 'typical' or 'average' speaker is rated around 90, I figure a system requiring half the power for the same output is high in sensitivity. 'High-sensitivity' goes WAY beyond that, however; speaker systems intended for home use with sensitivities approaching 110dB are not uncommon.

The higher the sensitivity, the less power the system needs for a given acoustical output. IF, and it's a BIG 'IF', an average system uses 90dB speakers and 100 Watts per channel and fills a room to satisfying levels, then with 93dB speakers one would need only 50 Watts, and with 96dB speakers only 25 Watts, and with 99dB speakers 13 Watts, etc.

An individual's need for loudness and dynamic 'liveness', and the room volume, are important factors in this complex issue. I'm using 97dB-sensitive speakers with 2.5WPC SETs on the midrange and am getting highly satisfying loudness and dynamics in a largish (3200CF) room while playing large-scale orchestral music. Yet some dideebopper may determine that my system doesn't get NEARLY loud enough since his ears aren't yet bleeding.

Perhaps a rule of thumb might be that if you want to use single-digit-power SETs, you'll need speaker sensitivity at least approaching 100dB.

I expect others will add more to this discussion.
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Through my experience, I would not match a typical low powered SET amplifier with any speaker less than 100+ db sensitivity. While the amps may drive the speakers, they most likely will not drive them as the speaker manufacturer intended. You may hear wonderful midrange, but bass and dynamics will suffer greatly.

Please also take into consideration room size. If you are in a very small room, you might be able to go with a little less sensitive speaker.

Most importantly is to select a speaker that has a benign impedance curve across the frequency range. This is true for any tube amplifier, not just SET amps.

Then depending on the rated output of an amp, you can look at the sensitivity number of the speaker. I use speaker with an 88db per meter sensitivity on a 12wpc rated amp. Most folks will recommend a slightly higher output amp, but I find it more than adequate.

Note, if your speaker does not have a benign impedance curve, no tube amplifier will drive them properly. Amplifier watts will not mask a bad speaker choice.

Regards
Paul

Suggest you go to Musical Fidelity's web site and read up on the subject. They have an outstanding article explaining how much power you really need to drive different speakers.