Having gone through all this soul searching looking for a means of adjusting the four voltages with one control, have you ever considered that the optimum carrier rejection is not achieved when the four voltages are all the same, but when they are slightly different due to the tolerance of the other components in the circuit?
I suspect this is why the original designer of the circuit used four controls.
JimB
I suspect this is why the original designer of the circuit used four controls.
JimB