There are mutliple reasons for this:
- Tube amplifiers produce more even harmonics than solid state amplifiers. Solid state amplifiers, on the other hand, are rich in odd order harmonics. For some evolutionary reason, our brains are wired so that even-order harmonics are more pleasant and warmer than odd-order harmonics. It’s something our body is biologically tuned to, so it’s impossible to change.
- Another reason is the way they are cut off. When overdriven tube amps are cut off gently, this sounds pretty smooth and soothing to our ears. On the other hand, when solid-state amplifiers are overdriven, they are hard trimmed, which is hard square wave cutting. It is rough and the quality of the grid sounds unattractive to our ears.
- Another reason is the euphonic distortion. These are distortions that occur in some nonlinear processes and sound pleasant or musically useful to human ears. With tube amps, it is third harmonic distortion that sounds pleasant to our ears for some reason. This again gives us the feeling that tube amplifiers sound warmer and richer.