I have just installed a pair of mains conditioners (Tacima).
I have one with all the switch mode PSUs in the HiFi/AV system, the other has all the kit with linear supplies, other than the power amp which is direct to the wall.
This has given as big an improvement as going from a Pi 1 with USB optical out to a 3+ Digione (I’ve not bothered to try the old kit with the filtering). The Apple TV 3 sounds a lot better too.
The pre-amp is is a Marantz AV9000