Question #1: did the music audible through the speakers match the music being received by the DAC? Whether or not it was the same will affect the diagnostic.
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This reminded me of something that really truly happens with my system. I don’t think it matters but I have a Denon AVR 4806 ~10 yrs old & we have outside speakers. Although no one is allowed to touch it, once a yr sound starts coming out of the outside volume switch. It’s very faint & you can feel the switch plate vibrating a little. A call to my stereo store & after some... push that, turn that switch all the way to the right & then flip that toggle down twice & once up... its gone. So, can some kind circuit or set of wires, in the volume switch, vibrate when you “cross the streams” & cause that sound? It’s usually the radio or what I’m trying to play outside from my iTv iTunes.
Well, you have either of two choices. It's coupled through the AC, or it's RFI/EMI coupled.
Try removing the interconnects altogether. They may be acting as antennas. Next, attempt to move the DAC closer and further away. You should get louder, or softer.
If neither has an effect, it's coupled through the AC power line. I suggest you split your devices up by either using multiple good strips like this Furman:
Or use a more advanced unit with multiple filter blocks. Furman, Shunyata and others.