Playing around with my Tourmaster and I realized that I really don't know what this is. When I switch the voicing on it I can hear the difference, but what is it doing in there?
In general, its a term of describing a tone characteristic in an amp. Voicing can be darker or brighter, more Marshally or more Fender-y, more British or more American, modern vs. vintage, etc.
I'm not sure what Bruce designed on the Tourmaster but there are things that can be done either through a simple change is voicing capacitor for the high end, or changing the slope resistor which will impart maybe a change fdrom Fender to Marshall type sound (In Fender the tone controls will have a greater effect whilst they have less effect in the Marshall mode)
Maybe someone with better knowledge will help out here (although as it's NAMM show this week most of the Egnater bods may be elsewhere)