I have two different outlets that are not connected, one has a Furman ac filter, the other has an APC H15.
From the Focal towers, it has KK PR8 cables, to Emotiva or Parasound Amplifiers, that are fed by the Furman, then Mogamy XLR to the Anthem AVM60 fed by the APC H15, on the Anthem I have CD player, Bluray player, Chromecast, and all that.
Changing the Amps, or the outlets does not help but I currently don't have multiple preamp and receivers available to test, but the hiss is noticeable only after a signal from the preamp is plugged into the amplifiers so either it comes from the wall or the preamplifier itself, but either I am super unlucky to get 3 or 4 different ones with some defect of something disturbs all the preamplifiers I had.