As mentioned, you can isolate with fiber, this is very cheap, and effective. I have had good luck putting a Stack Audio Smoothlan in between the router and my streamer.
Audio Grade Ethernet Cable
I've been thinking about getting a DH Labs ethernet cable. My thought is to get my crappy router as close to my Bryston BDP 3 as possible, so that the cable is only 0.5 meters. I do have a TeraDak LPS connected to the router. Is this a waste of time and money? I know the cable is just a couple hundred dollars but I don't want to spend that much just to realize again that my local files will always be preferable.