Matching speaker to amplifier?

Impedance and sensitivity, in my opinion are less important than phase angle of the load across frequency.
Tube amps do not like certain reactive loads. Some SS won't produce the goods, either.

If a speaker were 100db sensitive, and had a huge phase angle at some frequencies, you'd need large amount of power....just to deliever the right amount of power to the load. If this were a nice horn than the 8watt SET which you may think would work.....won't like the load.

Some speakers, like the LS 3/5 series are considered tube friendly and are pretty low in sensitivity.

look at the speakers impedance...is it flat and stable at 8 ohms or does it look like a ski jump dropping down to 2 and up above eight? crazy curves will like high current ss amps that "double down" as their impedance drops while you can run a low power tube rig on something that's 8 ohm stable. efficiency comes into play but not as much as the impedance imho

My default position on this kind of question is as follows:

I highly recommend the white papers on the website of Atma-sphere, the amplifier manufacturer called Debunking Common Power Amplifier Myths and Voltage and Power Paradigms. The pair of white papers, taken together, are very interesting and highly instructional.

Years ago I asked ProAC which amps they used to test there speakers. Would not tell me, as they felt it was a matter for MI15, or a break in to the factory.