Just going by the test point data sheet for the signals at TP2, apparently when the "Dirty" channel is selected (by plugging in effects devices and/or a foot switch, or switching channels from "clean" to "dirty", or all, didn't look all that close) the output of V1b goes to null. Since the tone controls are at the front end (grid input) side of V1b, it would appear that the tone controls (as part of V1b's circuit) are electrically isolated from the signal path.