There has been a resurgence of tube amplifiers in the past few years. The main reason is that Russia and China never stopped producing tubes. These tubes are entering the market and creating a new source for tubes. Tube amplifiers are said to sound more musical. The reason is that tube amplifiers produce even ordered harmonics.
Musical instruments give off harmonics in even orders.
Transistor amplifiers tend to give off harmonics that are odd ordered. These harmonics are not pleasing to the ear as second order harmonics are. Modern solid state amplifiers have very low distortions but their distortions are less tolerated by the ear than even ordered harmonics. Subwoofer amplifiers are particularly bad at creating odd ordered harmonics. I believe that the best tube and solid state amplifiers sound amazingly alike.