Just recently developed some subtle, yet ultimately crippling hum problems. Specifically, my Seventh Circle preamps and my Demeter preamps are both humming a bit, 60 Hz style, and as I turn up the output levels, the hum increases. Even when I unplug inputs into these humming preamps, the hum remains. I suspect it's a ground differential issue with these two boxes, perhaps in conflict with my interface (MOTU 896HD), but I'm far from an electrical expert. FWIW, I borrowed my neighbor's API lunchbox which doesn't have that hum problem. Any suggestions on how to fully diagnose this issue? I'm thinking of creating a very simple signal chain (preamps into Mackie Big Knob into headphones) and working from there, rather than troubleshooting with my existing complex signal path. Make sense?