I agree that the common 10K pot is the problem, but I'd like to avoid having four pots each of which have to be individually configured. I'm making this device (it's a little theremin/synthesizer) for my sister, so I'd like to keep the control set simple.
Since I've put the LED/photoresistor combo inside the box, it's pretty stable. So I could maybe put internal screw-driven pots for each input, and implement the remaining calibration dynamically in software... but that's hardly an elegant solution. Any other ideas?