Rick's saying in series with the LED, not the LDR
which would limit the brightness of the LED, which I'm thinking would serve as a depth control, but would also decrease max volume as the resistance was increased because the LED wouldn't reach max brightness. But then again, it all depends on just how sensitive the circuit is ... i.e. at what brightness is most of the signal getting through.
what if the guitar signal were jumpered from input to output and the LDR was placed between the signal and ground. That way, you'd have max signal when the LED is dark, and the signal would be bled off to GND when the led lit. If you added the proposed depth control here, the volume of the output signal would be unaffected.
and I'm just thinking out loud here too ...
cool circuit Rob