Tubes in audio are an oddity. They clip gently, which is a major factor
when driving amps hard, but it's also true that with the right tubes they
induce an odd sort of distortion that seems to match the hearing curve of
listeners. In other words, they sound more like you're listening to live music
than transistors, even though they don't measure flat. It's not an illusion;
very untrained people can instantly hear the difference, and the high end is
handled much better than transistors. They don't, however, go very deep, so in
high end audio installations you generally find things biamped, with everything
down to about 100-120 going to the tubes and under that to a tight transistor
amp. In a guitar amp, though, I'd suspect that the big thing is the soft and
gentle clipping.