The master volume is wired "backwards" on the schematic. You should wire them as a normal volume pot. The wiper should connect to R35/R36 and the top of the pots to C17/C18.
One more thing - I would recommend deriving the heater elevation reference voltage from the second (B) or third (C) power supply node. The first node (A) is a very "dirty" voltage, so it's better to start with one of the "cleaner" nodes.
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