As you stated, a dedicated line can be contaminated at the service panel even if it’s on a different leg than other AC devices. The problem with a subpanel is that it’s tapped off the main panel. The ultimate subpanel would have it’s own line run from outside and its own ground. But that wouldn't be Kosher.
Single plug dedicated circuit?
I guess if you truly want to do a dedicated circuit wouldn't you have to be allowed to only plug-in one device. If you plug two items into the outlet isn't one item going to contaminate the other item with noise from the first item?
So a multi branch circuit would be sharing 4 items.
I guess it gets down to sharing and Best to share with one but tolerable and acceptable to share with 4 components plugged into the quad outlet?
So how many people are only using one component per circuit breaker which is the truly finest way to receive a pure signal, at least if you ignore what occurred before the current reached the circuit breaker.