My first experience with a LAN filter was this one, made in South Korea:
LNF-C8G : 1000BASE-T LAN Noise Filter for Music Server. High Quality Capacitor(No Acoustic Vibration) : Ultra Low Distortion.
www.ebay.com
The performance graphs at the bottom of the page are interesting. I used it on the end of a 50-foot length of CAT8 cable, from a Netgear switch to my EtherRegen. I could not listen to the system with CAT6 or CAT8 alone once I had heard the filter's effect.
Later, I replaced the long run of ethernet cable with a fiber optic system and moved the filter to the LAN output of my router, feeding a Silent Angel N8. This placed it out of my main listening chain, which was from my NAS to the N8. I was not listening to internet music through the main system.
I decided to get serious about internet music and subscribed to Qobuz, but the sound needed work. I borrowed an iFi Audio LAN iPurifier from a friend, and a Network Acoustics Muon from another friend. Initially I tried the Muon between my EtherRegen and streamer. It compromised the sound by softening transients, and added nothing useful. I relocated it to the router's LAN output, feeding the N8, and it did a great job there. The NAS still sounded slightly better, but I was finally able to listen happily to Qobuz.
In the end, I went with the LAN iPurifier on the router WAN input, and the modified eBay filter (replaced the captive cable with a Furutech LAN 8 NCF) from the router to the N8. In other words, a LAN filter on either side of the router. Bass is a bit loose on the eBay filter, but the highs are just right. The iPurifier has tight bass but the top end is a bit bright and can be sibilant. Combining the two matched the performance of the Muon, for far less money, and that is where I am now.