You can play any instrument too loud. But sometimes this is necessary to do briefly or not so briefly when it's called for in a performance. In these cases the instrument distorts a bit, giving the sensation of increased loudness and drama. In these cases, the distortion serves the music.
Tubes do the same thing. Unlike SS, they distort a bit more at musical peaks giving a subtle sensation of liveliness. It's just the right amount of wrong. It's not accurate but it's fun. Who doesn't want to have fun...