I have both tube, transisitor-based solid state gear, as well as PWM in the form of a number of Hypex amps. The Hypex, as demonstrated on this site, measure the best of the bunch with minimal amounts of distortion and significant amounts of power. I am aware that the EL34 amp has far more distortion and far less power than my Harmon Kardon or Hypex amps.
After years of listening, my brain and ears tell me that I enjoy listening to my tube amp more than the other two, when using the same speakers and digital source. I am willing to admit that my preference may well be due to the form or nature of the distortion, or the differnece in harmonics. But I like it none the less, and I am fine with that. I listen to music for enjoyment first of all, and not necessarily for a component's measurements, though I want a competently designed and produced product as well. And I like all my systems, but not to the same degree.
At his 80th birthday party in NYC, which I had the fortune of being part of, I asked the late Les Paul what type of guitar amp her preferred. He was emphatic about his use of tube amps. I have no idea what his hearing was like at that age. But people like what they like for manifold reasons.
I have heard a mega system with Boulder amp and preamp (impeccable in every way) driving $50,000 Focal speakers that gve me listener fatigue within minutes. I am pretty certain it measured well. Maybe the room was bad, or the CD. But I would have been happier I think with some inexpensive gear positively reviewed on this site and priced for mere mortals.
Nelson Pass devised a small circuit he dubbed the H2 Harmonic generator, with which one could adjust phase and harmonics. Of it he said: "Well, it's a subtle thing. I don't suppose everyone can hear it, and fewer particularly care, but from listening tests we learn that there is a tendency to interpret negative phase 2nd as giving a deeper soundstage and improved localization than otherwise. Positive phase seems to put the instruments and vocals closer and a little more in-your-face with enhanced detail.
"Your results may vary, but when I first explored this with the SIT-1 amplifier at First Watt, I had a knob on the front of the amplifier which varied the amount and phase of the 2nd harmonic. It was easy enough to lend the amplifiers to listeners who didn't know what the knob did and gather their comments. Roughly speaking, they tended to prefer about 1% negative phase 2nd harmonic, so it became my standard setting for that knob.
"Of course we understand that all this is an illusion, like most of the rest of audio perception - you are welcome to take my remarks as entertainment."
In the end it is just entertainment and illusion. Can one illusion sound better to a specific set of ears if it has more distortion or 2nd harmonic? I am thinking that is so. And some people want to watch their TV's on color and contrast settings that are not true to the original.