It could also be the headphones with the grounding issue. I would suggest you replicate the situation where you are hearing the buzz through the headphones. Record a min or two of it where you are monitoring it through the headphones while touching the gain knobs and getting the buzz. Then, unplug the headphones, hit record again, and touch the knobs again the same way for a min or so.
Then, playback both files to tell if you can hear the buzz. If you don't hear it at all on either file take, or you only hear it on the first file where you monitored with the headphones, you have your culprit.