You will end up messing the EQ. The circuit depends on the high impedance of V3 to work properly. Also, this node is nowhere near ground. The dc output from V2 provides the dc bias for V3.
To install a pot, you'd have to capacitively couple to the attenuator (pot), then capacitively couple again up to a newly biased V3. All of which load down the EQ and mess it up.
Frankly, there is no good way to do this in a Cornet.
jh
