It is not a ground loop hum, more of a buzzing noise.
I only hear it with the 1030 GPU when JRiver is in theater view.
Hear it WHERE, exactly? And not the 5-feet-away answer, the ear-pressed-up-against-it answer.
You said before it was coming from your subwoofer. Be exact. Because now you are saying you still here it even when the HDMI cable is disconnected, but you also say it only occurs when MC is in theater view.
So if the PC is not connected to your receiver, then the hum is NOT being transmitted over the HDMI cable.
Does the hum occur if you disconnect the cable going into the sub? This is crucial!These are the only possibilities that remain if the PC is provoking the sound even when not connected:
1. You are mistaken about the source of the sound. Hopefully not.
2. The PC is leaking noise into the electrical circuit that is being picked up and reproduced by the receiver or sub or minidsp. If it is the sub, the noise will continue when cable to the sub is disconnected. This indicates bad electrical design of the component, which might be mitigated by a power filter.
3. The PC is leaking EMF radiation that is being picked up and converted to a hum by the receiver or sub or minidsp. This would mean the pc or component exceeds FCC limits and should not be used. You could build a Faraday cage around it to block this.
Please do some more test isolation of your equipment by strategically disconnecting cables.