Well the OP’s electrical noise almost certainly is coming through the USB connection as their DAC has no external power supply. Extremely common.
Your problem of an AC power supply not sufficiently filtering out high-frequency noise from mains is exceptionally rare to the point that yes, I also don’t believe that was the correct diagnosis of your issue.
Your problem of an AC power supply not sufficiently filtering out high-frequency noise from mains is exceptionally rare to the point that yes, I also don’t believe that was the correct diagnosis of your issue.