I've been looking into this, too, and the number of preamp tubes also seems to have a lot to do with an amp's sound. Typical 'Fender clean' circuits have one 12ax7, while Vox/Marshall circuits may have 3; from what I gather, this makes for more 'gain stages' and so an amp with less clean headroom and meant ot be more 'gainy'. Please feel free to correct me if I have any of that wrong!