[SpookyZalost]

So I did figure out how the circuit works.

The Integrated Circuits on the board are 4558's which use this circuit.



that's the chip diagram, if you look at the circuit it makes sense.




Considering that's the underside of the board the diagram would be flipped.

the power leads are connected to pins 8 and 4 and the inputs and outputs are setup in an amplification loop which outputs to the wire leading to the potentiometers on the front.

the resistors create an inverse frequency to the one you're cancelling out rather than filtering it similar to how mufflers and helicopter noise cancellation works.

it's very much analog and it obviously worked well given people swear by this make and model for it's era in any reviews I've found.

think of it like how noise cancellation works by trapping sound so it doesn't echo, only with it setup to bounce certain frequencies filtering them out.

it's actually kinda neat albeit not the standard approach which is just separating it out into separate channels and filtering the various frequencies then re-merging them instead.

I should be ready to order parts and begin proper maint and repairs soon so I can create a video demonstrating all this.