Have several knobs which are sampled via A/D conversion. This pretty much necessitates the 16F870. Knobs are much better than pushbuttons because 1) you don't need an LED readout, and 2) it's a lot easier to change an arbitrary setting in the heat of things, and 3) there's an implicit "off" to them.
Power supply ... 9v or 2x1.5v, but how to get the exact desired voltage?
Power button — simply cuts power to the entire thing, probably at the power supply level
Channel-powered indicators — two LEDs which indicate when the intensity is high enough to have turned on that circuit (2 pins)
Channel-intensity indicators — two (or just one?) LEDs which indicate in real-time when the channel is actually outputing electricity. In addition, it can be pseudo-PWM'd to indicate a relative intensity. This seems kinda odd though, you know what intensity it's gonna be. (1-2 pins).
Interrupt buttons — Presumably you can set the knobs to be continuous, and then interrupt them with the pushbuttons. (2 pins)
Output — PWM output from PIC is combined via a transistor with a reference output voltage (one for each channel) to increase/decrease the intensity. The output of that is possibly interrupted by the pushbutton, and then goes on to two independant step-up transformers which produce the maximum desired voltage. These go to jacks similar to headphone jacks, which various toys plug into.