Years ago my Paps used to go to the same phone repair guy for all his needs. He sent my Paps home with what he thought was a broken UPS. So Dad said I could have it if I wanted to try and fix it. I opened it up later and saw the huge transformer and cars, etc and realized this was a line conditioner. More specifically an Ametek PowerVar. It was the medical version. I took it home and put it behind my listening rig, and it did in fact make my silence blacker, and felt clearer overall.
But my entire apartment is on a single 20A circuit so my place was a true candidate for a unit like this.