Fully normal. You run an ungrounded system with leakage current that can't go anywhere. The moment you offer yourself as ground it can flow over the phones and your body. Has been discussed and documented in this forum already.

Typical AUDEZE and other planar magnetic ‘phones problem.

Indeed an extra ground wire connected to a screw of ADI-2 is the remedy, somthing like this:
https://www.conrad.com/p/wolfgang-warmb … et-1337382

To clarify - you removed the coaxial connection. That is a different grounding issue than the one discussed in this thread. The above connector might help - or not, hard to predict in that case.

Indeed ADI-2’s SPDIF Coax connector is linked to chassis/ground.

Worth to mention:
This is not in general the case with other devices (isolated and grounded Coax both exist), so it’s a bit of luck what happens.

• Coax can, but mustn’t connect to ground, hence may cause ground loop hum, or can solve hum caused by lack of grounding.

• Optical reliably isolates in case of ground loop related hum.

So, in case of hum problems, switching between Optical<>Coax is worth to try.
BTW: normal cheap audio cables fully work for SPDIF Coax.

That‘s good luck.
Or maybe a secondary connection like network or such?

Those who need it can find their simple solution above.