There is more than "hearing" alone. It has to do with the way signals
clip in vacuum tube based amplifiers.
They tend to do so more gracefully and organically. They naturally
compress a signal rather than just going straight from signal to
square wave.
Vacuum tube stuff is closely related to the physics in RF (radio
frequency) stuff. Take a look at the old books on the technology.
It's intense stuff.
Companies still do make tube-based audio equipment for precisely the
performance qualities they exhibit. Not only guitar amplifiers and
audio-phile equipment, also studio recording equipment and sound test
equipment as well as high-power amplification equipment (radio,
radar, microwave transmission)
consumer tubes used to have a bad reputation, but mil-spec (military
grade) tubes last a long time and do their jobs well.
If you can't hear the difference, I can understand with a stereo, but
ask any musician, they know and will tell you. They generally do
sound better but not always. Every component matters. Certain
capacitor materials, wire guages, wood used in cabinets, etc...
it all matters.
Much like choosing the right library for a parser.
