as ever, the thing to do is devise an experiment
try this - select a sound (strat edge 045 seems decent)
change the cab model for 'custom' on screen 1 of the VG
select 'twin' as the model on page 2
go to page three and check everything is at zero,
set bass middle and treble to 50 on page 4 and presence to 0. gain at 100 so we can listen to the edge and bottom
pick a parameter and vary it.
I chose the treble freq and the treble control. when the treble freq is low it, the treble control sounds more like a high mid control to me. (and yes my bad - i mistook 'high' for 'treble'
, that will teach me to do this at work instead of in my studio)
So what do you think it does? I'm wondering if it is an overall 'pre-eq' eq meant to model differences in the preamp, the kind you might get by changing electrical components or components aging .. pink noise analysis would help here as Elantric says