Of course tubes have been used as the output devices in power amplifiers since 1926 when the Radiola was first launched and, until the 1970s not much had changed. Were they the best for connecting a power supply to a loudspeaker? The question is irrelevant because up until the advent of the transistor we hadn't any other means to do so. But because we have only one way to accomplish something doesn't mean it's the best.
At a recent meeting of the Colorado Audio Society I gave a talk where I ate some crow. I had sworn an oath to never put a tube in a PS Audio product and now that has changed. It's not that I do not like the sound of tubes, I always have, I just never felt their frailties were worth their sonic virtues, and I was convinced we could do as well without them. I was wrong. For the purposes of voltage amplification nothing I have yet heard has been better than a vacuum tube.
Tubes do not work well as the output stage of a power amp, and it is not difficult to see why. For one thing they are not happy with the demands placed on them by the loudspeaker's wildly varying impedance swings. For another they require a transformer interface to protect them from the loudspeaker. Certain topologies of output tube amplifiers do not need such interfaces to survive but they are rare and stressed in other ways as their compromise. Tubes are most happy when they amplify voltages.
So why are there so many all-tube amplifiers on the market? Because they sound very musical in spite of their shortcomings, or perhaps because of them. But I have concluded the musicality of those designs is derived from their input stage tube, and the weakness of their output stage is not bad enough to mask those benefits. But they could be better.
When it comes to manhandling a loudspeaker, that is best left to devices invented by Brattain, Bardeen and Shockley; inventors of the transistor.
