"If you have the power amp as reviewed in Stereophile ... I would pay particular attention to its output impedance."
Excellent advice, and excellent subsequent explanation of that advice.
From time to time we come across someone dropping a high-output-impedance tube amp into their existing system, getting poor results, and predictably blaming the amp. A closer look will show that there was a mismatch between amp and speakers, often along exactly the lines Bob pointed out, wherein the speaker's impedance curve is wrong for that type of amplifier. This does of course reduce the speaker choices available for such an amplifier, but those who have taken care to match such amps with appropriate speakers are usually quite happy with the results.
see some fairly flat impedance curves take a look at my web page which
shows some of my designs impedance curves verses other name brand
models. Google any loudspeaker and see if they have a Stereophile
review and check out the curves of them. Unless they use series
crossovers, standard crossover impedance curves usually look like a
mountain range with large peaks and valleys. With a tube amp, this will
result in an uneven frequency response with bumps and suck-outs due to
the fact that tube amps will deliver more power to the frequencies with
the higher impedance peaks.
Disclaimer- I'm Fritz, the builder of FritzSpeakers