Unfortunately, different outlets don't necessarily mean different circuits but I would think if you're playing in different venues every week, sooner or later later you'll run into a building where that's the case and your problem would go away. Do you check your outlets in these venues? I'm not sure what part of Ohio you're from, but I can say from experience that a lot of buildings in downtown Cleveland have really sketchy wiring. I've learned the hard way to check for open grounds, reversed polarity, and in-range voltages before plugging any gear in.
Is either the relay pack or the control panel of your SC-8FC plugged into anything that's also connected to sound equipment? This includes your mixer, power amp, etc. Have you tried moving the lights around on the relay pack channels to see if it's actually the light or just the channel that's causing the problem? Have you just tried plugging the light into the wall and seeing if that causes the pop as well?
I can see in theory how a capacitor or flyback diode could terminate electrical noise heard through a speaker, but I would be super hesitant to try this myself, at least with parts from my local RadioShack. The type you would need would be very specific to the noise characteristic you're experiencing. I'm not aware of any off the shelf solution beyond ground loop destroyers if that's what you're asking. The funny thing is that LED units hardly have a surge when they turn on, and certainly nothing compared to halogen and discharge units! This sounds to me like there's some kind of filtering problem somewhere. If I may ask, what are you using for sound equipment?