It's some filters from H-N (and maybe H-G and G-N). They can be effective if you have dirty power on the specific AC line (from panel to your outlet, and any other outlets on that line).
But before you go there, it's probably just as easy to remove items that can cause AC line noise from that line (LED lamps, dimmers, poorly designed switch-mode power supplies, and God Forbid if you use things like Internet over AC or X10 home automation devices). Get that stuff off your HiFi line, and don't use X10 at all.
Depending on how your power gets to your home from the pole, even just your neighbour using X10 stuff can cause noise on your AC system.
If you need a lamp on that line, always use a good old incandescent lamp, low wattage bulbs are easily available.
If you want to test for such noise, play something at a reasonably high volume (not too loud), and hit pause / stop or lift the cartridge from your turntable. Then plug in a Hair Dryer into an outlet on the same line and turn it on while you are close enough to the speakers to listen for noise. You can also try the same thing on another line that is separate from your HiFi AC outlet.
You can buy a similar (identical?) device for $50 (or less, Amazon sells one for $20) if you feel the need. If your line is really bad (some people living in apartments, especially large cities, won't be able to keep the line clean just by cleaning up their own outlets) there are bona fide EMI filters that cost more (but not $2000) and do a better job. Even a simple Tripp-Lite ISOBAR outlet will help (and will carry 13A).
Often the best option is to use the filter near the offending item (same outlet). For example with inductive motor loads (furnaces, refrigerators, air conditioners) and other devices with interference potential (microwave ovens, WiFi routers) plug the filter in the same outlet as the offending device.
Or just don't listen to your system when the women in your house are getting ready to go somewhere.
