Denf; 12 awg THHN is just regular 12 gauge stranded copper wire. Each outlet required a hot, neutral, and ground of the THHN wire. It's used for running in conduit. The electrical company supplied it and it's pretty cheap-- any electrical supply store should have it. As my sub-main breaker box is on an outside wall, the THHN wires had to be put in plastic conduit against the outside wall of my listening room.
If your listening room is 50 ft. from your main breaker panel, you might consider having a sub-main breaker panel put in (as I did), and then run dedicated wires from the sub-main to each outlet. The downside of this is that the sub-main panel and conduit on the outside of the house is sort of ugly. Fortunately mine is in the backyard.
Avnut brings up a good point, ie you don't want a "colored wire". However, it's been my (limited) experience that any wire has it's own sonic signature, ie I found 12/3 Romex between main and sub-main to be sort of soft and uninvolving, whereas the big Zaner 6/3 stranded was much more live and dynamic when run the correct direction-- but it was also a little bright.
Lak (in the archives) compared 4 different "house" wires including regular Romex and Virtual Dynamics 10/4 cryo, and was not able to hear a difference among them, and I respect his experience. In my case, I think the old Zaner 6/3 was just inherently bright, and replacing it w/ the VD 10/4 (very live, natural, and relaxed, but IMO neutral)corrected a problem in my ded. AC system. Let us know how your ded. AC system works out. Cheers. Craig