Tube interference from router

How far away is the router??

Yes absolutely I was getting noise in my system a few years ago realized it was my modem/router. I moved it one shelf higher on my rack problem solved.

EMI/RFI certainly a problem. I have Trifield Meter, you'd be amazed how much RFI generated by transformers. Shielding and proper placement of components can greatly affect system noise. Some manufacturers take this issue seriously and provide shielding around noise generating components within equipment.

yes this can definitely happen ... play with moving a and b farther apart

Yes I experienced this with eero. Phono stages are especially sensitive to this - they’ll pick up and amplify this garbage and you will hear it thru your speakers. Just run a longer Ethernet cable and move your router away from components and cables.