I want to comment on jji666's really thoughtful post earlier this morning, in particular his mention of RFI and EMI "contamination" of the digital signal -- to which I wish to add "common-mode noise" as a further corrupting contributor to that signal. Earlier this month, I was fortunate to obtain a very high-end DAC, a Bricasti M21, with its own internal media renderer, ultra-high-precision internal clock, etc. (heretofore, I was using a Lumin T2 streamer/DAC, which was good but hardly spectacular). Immediately, I was amazed by how much better the Quboz- and Tidal-streamed music sounded, and this was while keeping my "old" Ethernet cable, which is generally considered to be fairly decent and cost me a little over $200.
A few days ago, I visited one of my two local high-end audio equipment stores and mentioned to the owner how delighted I was with my new DAC. He asked what cable I was using; when I told him, he excused himself, and, a couple of minutes later, he came back with a cable that he said would make a real difference in my system. It was Shunyata's Sigma V2 Ethernet cable, which he just gave to me (with no deposit) to try for a bit and see whether it made a difference. What is unique about this Ethernet cable is the two noise filters embedded within it. That evening, I replaced my own cable with this Sigma one, and my jaw dropped. The most immediate improvements were vastly more clarity and much better-defined bass. I might best describe the clarity is by saying that I could hear elements of the music that had previously been obscured. The next night I begged my darling wife. who is totally uninterested in my audiophile hobby despite the fact that before we were married she was a professional singer, to listen to the effect of this new cable. First, I played a track with my old cable and then with the Sigma one (without telling her which was which). She said she definitely preferred the "second one". Then I switched the order and played "Layla" (she adores Eric Clapton's guitar work). About 10 seconds in -- with the old cable, after first hearing Layla with the Sigma cable -- she called out "Stop! I can't listen to this version any further. It sounds DEAD." I switched back to the Sigma cable, and we spent the next hour or so playing a number of her other favorites, during which she repeatedly mentioned that they sounded "real" and "lifelike". Over breakfast yesterday morning, she asked when I was buying this Shunyata Ethernet cable. When I told her it cost $2,000 MSRP, she didn't bat an eye, pointing out that I had just spent $16,000 on a new DAC and that money would be wasted without this Sigma cable. I was nearly dumbfounded hearing that from her (but I bought the cable that same morning).
I still believe that most Ethernet cables -- whether a good cable like my predecessor version or one costing thousands of dollars -- perform similarly. What makes the Sigma cable different (even from the other Shunyata Ethernet cables except for their Omega one) is its embedded noise filters. RFI, EMI, and common-mode are real, and they negatively impact what you hear in the playback of a music stream. This Shunyata cable is insanely expensive, but it certainly does work. However, I understand there are very moderately priced line filters from companies such as iFi that I urge everyone to try in their system at least as a start.