Quad 34 setting higher outut level than standard 0,5Volts

I want to gain output level for an Quad 34 preamplifier. Seems to be pretty straightforward with this attached guide.
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Simply change R118 and 121 for a short and put 3,3Kohs in R119 and 122. This done to be able to have the right inputlevel for a old Quad 2 who likes 1,6volts . I also have a bunch of 33s and others that reqire 0,5V.
I have seen others set a potentiometer in the back of the 34 to be able to change output settings. Anyone now how to do this mod?