Check to ensure that the circuit the outlet in question is on the same phase/leg back at the service panel.
If it's not, hire an electrician to carefully move the circuits around so that all of your audio outlets/lines/circuits are on the same phase/leg in the service panel.
I had a small transformer hum in my amp and doing the above resolved that issue.
Of course, that does not mean it works every time. But it never hurts to arrange your audio circuits this way anyway.
-IMO
If it's not, hire an electrician to carefully move the circuits around so that all of your audio outlets/lines/circuits are on the same phase/leg in the service panel.
I had a small transformer hum in my amp and doing the above resolved that issue.
Of course, that does not mean it works every time. But it never hurts to arrange your audio circuits this way anyway.
-IMO