A wild guess, but the neon lights might be your problem. I know they are trouble for my guitar amp. When we have play clubs with them, you always get a nice hum from the amp. They interfere with the pick-ups on the guitar. Could be some kind of power surge to them, that is being picked up by your amp. Since that is one of the only additions you have made since your system worked fine, then they might be the problem. Disconnect the lights and test it.
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