Pics of said "sole" knob? What amplifier is this on?
Sole, as defined as "only" could be a slave out level, master volume mod, reverb, presence, effects loop send or receive level...
Sole, presumably "Soul" could be a negative feedback knob. This adjusts the amount of negative feedback reintroduced upstream in the output circuit. Less FB--a more open, gainy, fat sound, more NFB--a cleaner, tighter tone.
The Sole knob could also be how much flat fish you want added to the amp.