Help to add trim pot to reduce gain of SD80

I want to reduce the overall gain on both channels. I have tried lower preamp tubes and Jeff Bober (has a new life, unable to assist). I need to know where to add it on the PCB and the trim pot value, plus any other component value tweaks/changes. This head is one of the first twenty built and has never been touched except for tubes and bias tweak. No schematic around and we don't want to experiment. Any help or referall to someone who knows this amp well is appreciated.