Get a group of audiophiles in a room - especially a bunch of guitar players - and ask their opinions of vacuum tube amplifiers or solid state amplifiers. And get ready for a really active discussion.
Let's back up a second. A certain amount of "WTF are you talking about?" is expected. When we say "amplifier", we're actually referring to a system of parts: a cabinet with a speaker or two that takes input from an audio preamp or maybe a guitar and brings it up to room-filling volume. But each one of those is based on an active amplifier device that takes small voltages (or powers...) and makes them into bigger voltages (or powers...). The active device that does this - the amplifier in the amplifier - could be silicon: a bipolar transistor, or a Field Effect Transistor. Or it could be a vacuum tube: metal electrodes, hot plasma and wire running in an evacuated glass envelope.
I've got to tell you, I've been paying attention to technical details on this subject for years and it is one of the most complex subjects I've ever come across. One of my frequent reads, Lou Frenzel in Electronic Design, dives into the topic. He ends up sounding like me.
I have actually compared solid state audio power amps to the vacuum tube equivalents several times and using the same speakers. (It seems to me that the speakers would have more of an effect on the sound than the type of amplifier.) I could discern a difference between the two. I do not have the words to describe the difference. It is akin to comparing wines in a tasting. There are words for that but they are also vague and subjective to be sure. So it is with audio sounds. I have actually heard people say they can tell the difference between two different sets of speaker cables and connectors. I still don’t believe it.There's a big deal in there, when he refers to the speakers having more effect on the sound than the type of amplifier. The guys who build these amplifiers frequently don't account for everything. Speakers couple magnetically into audio wires; wires interact with each other. Moving the input transformer with respect to the input circuit can change the way the system sounds. Maybe it's all the effects of the transformer and has nothing to do with the active devices (transistor amplifiers tend to drive the speakers directly while vacuum tubes need the transformers).
One of the most common things you'll hear is that transistors tend to odd order distortions - odd harmonics of the fundamental tones you produce by playing a string - while tubes tend toward even order distortions. The theory goes that humans perceive even harmonics as sounding nicer than odd harmonics sounds. I recall back in my days playing with electrical things as a kid running both a square wave and a smooth sine wave into a speaker: the square (odd harmonics) did indeed sound harsher.
I'll be 100% honest, as always. I don't know what to make of this. Our ears, our "psycho-acoustic" processing is amazing. But audio is ferociously complicated. Everyone who has played around with stereos has heard of "8 ohm speakers", but there is nothing inherently 8 ohms about any speaker, and the actual impedance measured at the input varies with where the speaker sits in the room. The back pressure from the speaker compressing the air in the room and that air pressing back will change the impedance. Our ears can hear amazing things, but so can modern instruments. I think the situation is that there is no simple, tidy little answer. It's not odd harmonics or even; it's not transformers or direct drive; it's not transformers perpendicular to their wires or not. It's all of this and more in a subtle blending. It's complicated.
Maybe the answer is get a box of tubes, some sheet metal, and the rest of the parts and put a few things together.