So you are using some type of Wi-Fi device? That changes the game.
1) Put the fiber section between the router and the Powerline device and see if that sounds different.
2) Try to run an ethernet cable from your router to your music server with and w/o the FMC’s in the chain- if you can to compare the sound.
I bought a long ethernet cable and ran it from my audio grade network switch, which is after the FMC’s, to my TV at the other end of the house. The picture looked so much better that I ended up working 6 hours running that cable through the walls and under the house to the TV. Running an ethernet cable to the 2nd mesh router at that end of the house made the TV picture just about as good but since I had to run one cable to that end of the house it wasn’t much more to run two.