No need for a sub panel. All you need to do is use a piggy back breaker in the existing panel, if you happen to be short on space. If enough room, then add a conventional 20 amp breaker and run 12/2 romex to your receptacle of choice. I chose Audioquest’s Edison receptacle. I believe in synergy, and so I also run a AQ Niagara 1200 into that receptacle along with AQ power cords. The sub panel will not make any difference In sound quality. If your hifi is on a seperate circuit as above, you are not going to get noise from an appliance on a seperate circuit. I don’t care what anyone else tells you here, it’s bull. My breaker for my hifi sits right next to a breaker that controls a fridge, no noise on the hifi circuit. You are making it way more complicated and expensive than it has to be. All you need is a 20 amp breaker that matches what you already have in the main panel, and a high quality hospital grade 20 amp receptacle, be it a home depot or Lowes product or an audiophile receptacle such as Audioquest, Furatech, or oyaide. Lastly, a run of 12/2 yellow romex...no need to run 10 gauge...too stiff and hard to work with. You want to get real fancy, add a RFI/EMF receptacle cover plate by Furatech.
Use Romex cable, unless you have a rat problem, then use metal Clad or conduit. This project is quite simple and not expensive. Black to breaker screw, white to neutral buss bar, ground to ground bus bar. Heck, I did it with the power on., not recommended. It helps that I took electricity in Vocational school. 😁