I usually like to have @jea48 in these threads because he really knows the NEC and your local code may add more.
On the sub panel issue, I did add a sub panel with a copper buss bar to set up what amounts to a separate subsystem for the hi-fi.* Additional connections notwithstanding, it was a fairly long run from the main panel and adjacent new sub panel up to my room. That was connected via 4 gauge feeder line (I had the electricians install a 10kVA isolation transformer) and the run via 4 gauge eventually terminates in another small sub panel in a room adjacent to the listening room, where 10 gauge Romex was used for multiple dedicated lines to hospital grade Porter Ports from good ol’ Albert (I don’t think he has them anymore). All ground tied back to the main household ufer, as others have mentioned.
They did have to break sheet rock to run the dedicated lines but it wasn’t that big a deal to patch and paint. FWIW, I had Romex in a conduit in a previous set up and it was a disaster- noise on one line (from an air compressor for my tone arm) was transmitted to the system given how the lines were bundled. I think each install presents its own challenges. Even if you are doing your own work in a jurisdiction that permits it, it might pay to get a consult from a solid electrician. I lucked out this time (not the first time I’ve done a dedicated room) and got some commercial electricians who were willing to do residential work as well. Went very smoothly- no issues since this latest installation in 2017.
I have always pulled permits for this stuff and where I am now-- Austin-- is permit crazy. I’ve seen wiring out in West Texas that was absolutely frightening, I figure out in the country, they probably have a guy who does triple duty- Justice of the Peace, funeral director and electrical inspector. :)
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*This will also help me cut out the hi-fi from the eventual install of a Generac to avoid the problem Fremer said he had with the ATS creating noise on his system even when the generator was not operating. I have no need to play the system when the main power is out, but I'll be able to route to the hi-fi subsystem first, then into the main service panel which will tie to the ATS for the generator system.