"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.