But alas it came back.

This sounds like a bad connection more than anything else. I had eliminated my whine by getting a new amp (none of the tutorials suggested this, it was a last ditch effort. I replaced JUST the amp and it went away). But a week later it came back. Ends up that I had loosened one of my crimps near a grounding point for the empeg and that was the cause. So when a ground "comes back" without you changing anything, I'd seriously check for a loose connection or bad solder/crimp.