I stared this thread as there is a lot of incorrect perceptions on the subject.
I agree SE was never the best answer, but he is talking about the physics, not if you need it. My point is, SE is probably just fine for a home system. Listen to see if it is, don't assume what is needed for 100 foot studio runs is also needed for 3 feet. Fix real problems you find in your system with the help of information from the actual field experts rather than the fad of what You-Tubers and audio parlors can sell you.
Yes, we have electrical building codes for a reason. Follow them. Yes, a lot of old equipment did not have safety grounds and the leakage could give you a tingle. Easy to fix. I like the JDS answer to use AC wall wart transformers. No ground loop and zero risk of fault to mains. Any competent mains powered unit will deal with ground isolation internally. It is not hard or expensive. Even I can. No excuse.
I only had one piece of equipment that had line induced issues. A Parasound amp so I dumped it as being incompetently designed. It also sounded like garbage. Not like the old 1200's. I do run a DC blocker as it filters DC added within my house by cheap devices not supplied by the power company. Real problem. Real fix.
One store I visit does play all kinds of games moving speakers within 1/4 inch. So what! I move my head more than that. If you have to hold your head that still, it is a useless system. ( Kef Meta's?) On the other hand the big store here has the wall of speakers driven through switching and a mid-range class D amp which are almost useless other than to eliminate the worst of the bad. They will take a pair down to a better room to their credit but that leaves you with no comparison and within seconds your brain remaps so still not a very valid audition. End result is, you can pick something to take home over the weekend and give areal listen.
R2R, I think you mean Reel to Reel low generation dups onto quarter track a few studios issued? All analog. ( In todays jargon, it is assumed to be a ladder DAC) Yea they were good but I only had a couple. Way out of my price range and were only available mail order. I had a 10 inch Teac. Like vinyl, they degraded with every play. I went for the direct-to-disk live performance route where I could afford them. And don't tell me MoodyBlues analog mastering quality is HALF of Redbook. Things got better with the second generation SONY digital mastering ,16 bit, then 18 and on. Yea, my CATs CD was mastered in 14 bits. Most early Deuchas Gramaphone was. No wonder the best vinyl sounded better. 40 years later, newest studio work can be very very good. Unfortunately most of my music collection predates digital recording and was re-mastered and processed digitally for CD release.
If your "audiophile Ethernet" sounds better to you, have fun. It is your perception in your system that matters to you. I am not a fan of made up problems that may exist in theory, but not in the real world. If network noise is causing you a problem, you need to find the faulty devices. We don't need audiophile grade switches for 10G, live fire, nuclear, life threat, and can bounce 100% reliable data around the world on the Sonnet ring and satellites. If you can't within your house, you got a problem. If you can identify the demo of a DAC being lackluster due to Ethernet noise, you should get some University to study your clairvoyance. I am not saying you did not hear a lack-luster DAC. You heard it, I did not. I suggest your reasoning is less than a guess. It could well be you are not used to a clean reproduction from a really good DAC? Or maybe it was Cambridge/Rotel/Arcam amp with half the power supply reserves needed not the DAC? Was the stream actually high quality? Most is not.
Setup, room, treatments, yea, duh! Differences almost as large as the speakers.
You missed my point on if high end gear is effected by noise; digital, ground loops, power, then yes, I blame the OEM. NO excuse. Apples will start rotting as that is the environment they live in, just as dirty power is the environment. We understand the environment of apples and learned to pick them before they fall off. Just as decent engineers know how to handle system noise.
If you play vinyl, then isolating your player in a sealed box does wonders. Put your RIAA preamp in the base of the table close to the arm and you won't have any RFI exposure. That way you can maintain the balanced signal from the cartridge until you have provided enough gain that a couple feet of coax is fine.