Meridian Audio discovered that RF, picked up by the Ethernet cable, could get into its product (ID40 card) and was unambiguously audible. In fact, they heard the difference first and it took them ages to realize that the audio quality improved when the Ethernet cable was removed. They then worked around the problem, initially in the Anniversary 808 by using RF suppressors on the cables, and subsequently with the development of the ID41 card which has much better E/M isolation.
So please don’t state as categoric fact that Ethernet cables don’t make a difference: they can, but not in the way that you imagined, and longer cables might pick up more RF. That’s not to say that you need to spend a fortune to resolve the problem – far from it – but well made and double-screened (screened twisted pair plus screened sheath) Ethernet cables are well worth considering.