But the dirty power happens outside the home
Actually not. The majority of it happens within the home.
and you can’t do anything about it with a dedicated line. Not most of the dirty power, but 100% of it so long as you have nothing else on the circuit.
If you mean a convenience outlet branch circuit. Unplugging everything from the wall outlets except the audio system does nothing if the in and out wiring connections are not solidly connected connections. It’s possible there might be 8, 10, 12, or more wall duplex receptacles on the branch circuit wiring. The OP’s audio system might be on the closest outlet to the electrical panel or the farthest one.
Like I said in an earlier post a poor wiring connection can/will add noise, harmonics, into the circuit wiring that will transfer to the audio equipment.
Three things needed for a good electrical connection.
Clean connection.
Surface area.
Contact pressure.
I have no idea what wiring method was used to makeup the connections. The worst, imo, would be cheapo residential grade stab in the back duplex outlets. Junk! (Not to be confused with a duplex receptacle that uses a terminal plate that is tightened down on the wire by the 10/32 side terminal screw. The stab in the back duplex outlets are notorious for making poor, bad, in and out connections. Every one of them adds noise. Don’t forget there is a hot and neutral conductor. Four connections per outlet.
Then, it’s not unusual to have ceiling lights on the same convenience outlet branch circuit. Maybe they are can lights. Lamps are screw in LED. Add all them noisy drivers to the branch circuit.
Example: Stab in the back wire connection.
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