The extra knob is a master volume for the JBE mode. I asked Dave to make mine foot-switchable between the Clean, BE, & JBE channels. Without an extra master for the JBE, the volumes would not match as the JBE mode lowers the volume because of the clipping...
My understanding is that Jerry only wanted a clean/dirty footswitch with the option to choose either BE or JBE only along with clean.