The Absolute Sound article posted by @chuck is very thorough and sounds right to my non-electrician ears. I wish it was available when I purpose built my listening room from the studs out 15 years ago, including the power lines from the breaker box. What I got right based on the article is a dedicated twisted 10 AWG twisted Romex line on a dedicated 20 amp circuit - complete with silver paste on the breaker and outlet contacts. What I wish I had done is running one or more additional lines for my audio and video equipment. Oh well, at least the phase is the same to all components.
Did this dedicated line make a difference? I think so. My system at the time was a modest powered integrated amp, a CDP, and some Totem tower speakers. Compared to my living room where this system was connected to a non-dedicated 15 amp 14 AWG line, when moved to my dedicated room the amplifier sounded both more relaxed and especially more dynamic. I would recommend using the highest gauge supply cable from the box that is practical, but nothing less than 10 gauge. One note, if you opt for 8 or 6 gauge and are forced to step down in the wall to 10 gauge to connect to your outlet, I believe code will require a junction box.
kn