Tubes sound better, (especially linear triode tubes) because they have less distortion than bipolar transistors or FETs. They might not have less distortion in circuit, but transistors need some kind of feedback and triode tubes do not. I also think the size of the device has something to do with it, I have noticed that larger tubes in preamps sound better to my ear than smaller devices, tubes included. This may have something to do with microphonics, the old globe style triodes sound better than coke bottle tubes of the same manufacturers.
