Sound quality differences between NAS units

[Mike48]

To summarize what I understand people to have said:

If a NAS introduces noise on the Ethernet line, and the noise is passed through by the (digital) renderer to the DAC, and the DAC doesn't filter it out, either, we hear some degradation of the sound? Is that the prevailing hypothesis to explain this observation? How does that relate to the inevitable other devices connected to the network? (If I remember a review of the Melco correctly, they advise connecting their NAS and the renderer together and to nothing else, but to me that is not a useful form of networked audio.)

If noise on Ethernet is a problem, would using WiFi eliminate the problem, at least in theory?

I'd like to understand what is being proposed here. Given the excellent sound quality I've heard extracted from music on a Synology NAS, I am dubious how big an issue this could be. However, audiophiles are interested in many changes that I might find to be second-order or third-order effects -- smaller than moving one's listening seat by 15 cm, for example. I have to agree with Win Xi that the biggest variable, at least in my case, is often the ear-brain system.