the question of changing ethernet cables

You are correct on noise coming into house, however self generated noise and jitter from modem on still exist in all streaming setups. Whether you need a filter or not is for you to discover via experimentation. I've done much experimentation with streaming setups over the years, sometimes there is progress other times regression, and sometimes change is just change. Streaming can be as simple or complex as we choose it to be, curiosity certainly has hold on me!

I purchased a Shunyata Sigma Ethernet cable that goes from the wall to my streamer.  It made a positive difference. I purchased an open box, so somebody didn’t keep the cable, so make sure that you have the ability to return the cable.

There are many comments here that fibre avoids EMI/RFI conversion altogether. Conversely some very experienced members prefer Ethernet avoiding the media conversion to fibre. I concur with @sns that any streaming solution is unique, depending among others on the quality of the incoming signal, quality of modem and router as well as their power supplies, EMI/RFI intensity of the location, quality of the cables as well as the receiving port implementation, noise rejection quality of server and dac and and and…l

In my experience using isolators on both Ethernet and USB resulted in significant quality improvement, that said YMMV

Unless you’re running fiber from your ‘box’ to ps audio streamer, you’re still susceptible to noise. In your setup, your router is a potential source of noise. Are you using a wall wart (another source of noise) or linear power supply to power the router? As suggested by @antigrunge2, use a network isolator ahead of PS Audio. Listen, access and share your thoughts.

Many thanks. I understand the issue better as being 2 distinct loops, shall we say. The problem I wish to solve is twofold: Firstly, now that I have the hardware, I wish to optimize the cabling and refine the system (mostly Cardas XLR). Secondly, I have a white noise (?) at the higher mids or from the mid range driver (Speakers Focal Sopra III).

To begin. I had Furutech fuses (Sicherungen) connected to the fuse box and Furutech outlets at the wall. These are 2-separate lines with 6mm2 (10 Guage) wiring. These are dedicated lines with no loop at all - direct. One line is just for the amp (PS Audio 250 sig) connected to the wall with a Shunyata Alpha NR. This has been a game changer.

I have read here and on other posts that the router and ethernet cables can induce some noise pick up. The first question has been answered; namely, despite the fibre hook up coming into the house, yes, noise can come from internal parts thereafter.

I am very impressed with Shunyata and I will look into their ethernet cable. I also saw a video and post about green computer or something like that using filters and adapters from ethernet to fibre connections. I believe the remaining hiss or white noise is coming from the router. I am almost ready to try this remedy. I just needed to clear up my quaetion. Lalitk and antigrunge2 sound like they are leaning in this direction. My plan would be to try this first and later, if necessary to replace the ethernet cable.

Oh, if I have just the amp and preamp on I have a very slight hiss, volume turned up and ear to speaker or just some inches away. When I turn the streamer on the same noise increases. I would like to have dead silence if that is possible. So, my plan is go one step at a time to reduce as much of this hiss as possible.