Here's an idea I've been playing with...
I recently 'discovered' how to make circular PCB features.
I used to make footprints for tubes by drawing a silkscreen circle, and then using trigonometry to calculate the positions for the pads to the nearest thou. It works, but it's cumbersome. Then it occurred to me I could use the 'block rotate' facility to do this. Draw a circle round the sheet center, place one pad at a known distance, select a symmetrical area round everything and rotate it, say 36 degrees. Repeat until 9 pins are placed.
So here we have 31 SMT resistive divider pairs arranged in a circle, all connected to 2 common tracks (blue) for signal and ground. Call this the faraway or bottom side. Then there are the (red, nearside, topside) connector pads and 'wiper' track on this side of the board, connected to the resistor pairs with tracks and vias. Now all we need is a rotating connector to connect the topside pads to the wiper track individually, preferably on a 'make-before-break basis'. You can use high-quality thin-film SMT resistors. The pads are designed to accept 1206 or 0805. You actually get 62 gain steps, but I/O impedance alternates on the half-steps, so you might not want to stop on the intermediate positions, although in most applications the I/O impedance is not critical.
Then we have additionally 2 stepper motors with a PIC microcontroller, all running off 5V. Pull up one pin on the uProc and the stepper rotates clockwise, pull up a different one, and it rotates anticlockwise. The stepper motor even comes with flats machined on the shaft. The motor fits on the matching square PCB and the disk slides onto the shaft
I've figured out a simple ballbearing make-before-break contactor, which can be mounted on the disk. 2 additional springloaded nonconducting rolling supports will keep the whole thing parallel. I say simple, but getting the contactors and other rollers manufactured is the biggest challenge in the whole project.
So now we can arrange all this with a digital (7 segment) readout, touch-sense buttons (a favourite trick of mine) for up/down and an infrared receiver for remote operation, with of course 2 attenuators for stereo. Or as many channels as required. With whatever taper you desire. Or, of course, you can build it as a linear balance control.
Kind of like this touch-sense/remote digipot-based 8-channel volume controller I built previously:-
...but with absolutely minimal distortion.
Add a bit of software to make it move in steps, recalibrate the position at switch-on and select left/right...
You can make almost all of it as PCB with a few stand-offs to hold it all together. The fascia can be laser engraved smoked acrylic. Side-firing LEDs shoot light into the edge of the fascia for illumination.
Of course, like anything else, the devil's in the detail. It's when you go to actually construct something like this that the true problems really emerge. What do you think? Worth pursuing?