The iFi AC iPurifier just arrived and going to give one a try. A $99 product. This looks interesting with the Active Noise Cancellation ideas supposedly employed. Also tests your AC line for standard connection problems.
For a system, they recommend 3, but should be able to hear something if it works with one.
Going to put mine before the PI Audio UberBUSS running the MS-3 Music Server and dspMusikLCD digital processor and see if anything changes with the noise floor.