[morzh]

Rich

Simply speaking, while producing harmonics, a tube amp produces mostly even ones.

A solid state makes very little, especially considering they usually have very deep feedbacks that bring them down.
But if they were to make some, they go into odd harmonics and they will sound unpleasant, unless brought very low (this is on high quality ones go down to 0.01% and lower).

Today's popular way to estimate quality is measuring THD+N, and noise dominates the low power, so THD+N goes down with solid state significantly up to 1W or so, and then stays mostly flat until reaching the clipping region.