Tube amps (as do transistor amps) can vary quite a bit in how much they change the sound, e.g. part of the appeal of tube guitar amps is that you can drive them out of the area they work "most precise" into distortion, and that distortion sounds interesting. And some people like the sound changes tube amps cause for listening.
There is an story that when the Beetle's went to record at the BBC there was an argument with the BBC sound recording Engineer that no way was he allowing distortion to go out on the BBC :-)
