Both Vacuum Tube (valve) and Transistor can be made for amplifier.
They have their own characteristic, advantage and shortcoming.
At the early 80’s, the development of digital audio is at the preliminary phase.
People started to use transistor amplifier for playing digital sound.
It really improved the dynamic range, definition and preventing from susceptible to noise.
However, there are still several problems occurred while playing the digital sound.
You feel the digital sound hard clipping and cold.
When you visually represent the digital sound wave in a graph,
you will see digital sound wave only replicates the sampled sections of the original sound wave.
The sound is unnatural.
Moreover, listening to such sound in a long time would feel tired and bored easily.
Yet, the vacuum tube and transformer worked perfectly to distort the digital sound wave slightly and turn the digital sound wave into analog sound wave.
Eventually, you feel the digital sound processed by the vacuum tube amplifier is more natural.
When you visually represent the sound wave processed by the vacuum tube amplifier in a graph,
you find that the digital sound is presented in analog sound wave which makes you feel the sound real, natural and pleasing.