This is baffling. I was connecting up a stereo keyboard using Aux and Guitar as left and right inputs, panned accordingly and recording via the tuner, Bypass, and USB. After having to turn up the volume, I realised an intrusive hum, somewhere around –65 dB, had gotten into my signal, both for monitoring and recording (read: in the inputs, not just outputs). Further experimentation showed that unplugging either of the inputs would get rid of the hum altogether in the one that remained, but as soon as they were both connected, the hum would reappear. It does seem slightly louder in one of them, however.
What could be happening here? I found some old threads suggesting a ground loop, but surely the way to cure that would be to disconnect the computer or to do something else that would remove an errant AC component from the network of devices. Why would disconnecting either of the cables have any effect upon a hypothetical ground loop?
All suggestions would be much appreciated. I need to go and test this a bit more with different combinations of stuff plugged in. And I guess I will check it in a spectrogram to see whether the hum is concentrated at 50 Hz or not. If I can post more info later, I will. Still, if anyone already recognises these symptoms from their own experience or can theorise about a cause, please let me know your thoughts.