> Vacuum-tube amplifiers are not in the same class with techniques that are unlikely to have any perceptible influence on what you hear.

I don't disagree. Not in the same class, but the audience overlaps.

> one could make a modern amplifier reproduce any quirky behavior of vacuum tubes, e.g. a higher and frequency-variable impedance or certain kinds of distortions, but usually nobody bothers to do this, because it would be expensive

In other words, the willingness to pay for OG tube amplifiers exceeds the willingness to pay for the sound thereof. I'm not sure you disagree with me either.

As a child, I have used some very good vacuum-tube audio amplifiers, which had been built by my father, at a time when they were still the cheapest solution, instead of being a luxury product.

They were excellent, so I feel nostalgia remembering them, and I would like to experience again listening through such an amplifier. Nevertheless, if I had so many thousands of $ to spare, I would rather buy some memory modules ... :-(

When I was young I made a few unusual transistor power audio amplifiers, e.g. with the output transistors biased in class A and designed for high output impedance instead of low output impedance. I was very satisfied with their sound and some of them resembled more some vacuum-tube amplifiers than typical transistor-based audio amplifiers.

However, despite their high audio quality they would have been completely impractical as commercial products, because they needed very big power supplies and they produced an enormous amount of heat. Semiconductor devices are much more difficult to cool than vacuum tubes, for the same amount of heat (because the temperature limit for the former is much lower than for the latter).

Nowadays, switching amplifiers can cover all the audio bandwidth with excellent energy efficiency. With an appropriate combination of linear and non-linear feedbacks, one could reproduce both the distortions and the output impedance of vacuum triodes, to make an amplifier hard to distinguish from true vacuum-tube amplifiers.