My Stargate has a slight hum also. Noticeable when you walk in the room but not while in play. I disconnected the coax from the aux sound board to the aux power supply board and shorted the audio input to the aux power supply board with a little wad of aluminum foil in the connector. Still got the hum - but not the power up "thunk". So my hum isn't from either of the sound boards. And the power-up "thunk", from what I've read on line, is due to the sound board D/As not being held in reset during power up. Back to the hum... Seemed to mainly be coming from the cabinet speaker. The cabinet speaker does not have a dedicated signal return - it runs to the center tap (CT) of the power transformer and the CT goes to the aux power supply. Not good practice to mix power and signal grounds so I ran a dedicated speaker cable form the aux power supply board to the cabinet speaker by splicing into the back box speaker signal return and the cabinet speaker output right at the cable connector going into the aux power supply board. See photo. Hum was significantly reduced but not eliminated. Can't hear it at all when you walk into the room until you're a few feet from it. Also completely masked when central air is on. Wish I had a spectrum analyzer to have more quantitative results to share.

Speaker Cable in Back Box (resized).jpg

Speaker Cable in Cabinet (resized).jpg

Glad to see this fix is working for others and that someone actually measured the improvement!