This is a great example of "worse is better". Unlike amplifiers made for listening to music, in a guitar amp you want that distortion and non-linearity, you wan

This is a great example of "worse is better". Unlike amplifiers made for listening to music, in a guitar amp you want that distortion and non-linearity, you want it to be wild and resonant and you want it to be sensitive to what you do on the input side. Not directly related to tubes, but see the enduring legacy and popularity of the Boss HM-2 distortion pedal in metal circles, very often employed with all the knobs turned to eleven for maximum crunch. It was used to great effect primarily by Swedish death metal bands such as Dismember, Entombed, At the Gates and their compatriots. The tone is rough and textured and just feels right for the genre, so much so that it is considered a clearly defining characteristic of that particular branch of death metal. Just like tube amps, it's preferred because it's imperfect in just the right ways to produce an artistic result.