well, gain and sustain, of course, but it seems like the mids change,too-mind you, my "mids" and someone else's could be a little different-i also notice more of that popular high gain hiss....maybe that is why i am mellowing in my elder years(more mid gain and pushed in these days...)
The pull shift on the treble pot adds the 250pf treble cap with a 750pf treble cap. It shifts the mids and is not really bright, but it is a shift in
the treble and mids. It only alters the lead channel.