Is everything powered from the same outlet / power strip? It does sound like a ground loop is present. I’d try and record something and verify whether the humming is present in the audio or if it’s related to the headphones/headphones output specifically.
I’m just hypothesizing, but from what you described, the problem is probably with the tracker, even though you tried a different USB power source, USB is notoriously noisy. Although the digitone seems like it should be the culprit, in all situations with humming the tracker is the common denominator.
Unless you have a whole other set of cables you can try, I’d start by running a few tests of connecting everything except the tracker, if there’s no humming that would be a more coherent indication that the problem is with the tracker.
If you have a in-line audio ground loop isolator, try that after the headphone out and see if the noise goes away. The reason I suggested a test recording is that if you are only getting ground loop interference in the headphone out, you may be able to work around it with a ground loop isolator. Also, though the chance is slim, don’t rule out the headphones themselves. It would not be consistent with your test results, but my noise cancelling headphones exacerbate noise in certain gear and not in others.
If things are plugged into 2 different outlets, do your best to bring it to 1 single outlet or power strip (that’s the simplest way you can hope to eliminate a ground loop if it’s as a result of 2 wall outlets with separate grounding).
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