Unless there was something fancy in the knob graphics that could make something go red (or whatever colour) when the knob passes 15db.
2) You could get around this dilemma by having two seperate modules - and basically copy/paste the code - except for the limits of the knobs.
Otherwise - like # 1 I'd say make it go +/- 15db, and put a blurb in the manual explaining hos stopping at +/-6db makes an accurate mastering model.
Just MHO

