Anyone compared Premium AC wire to regular romex?

I should have added that I think the dedicated AC system for your stereo gear is much more important than the type of wire used. I suspect that dedicated 12/3 Romex would be fine, and a huge improvement over plugging your stereo gear into the general house AC system.

Is it possible to add another, seperate breaker box, say 20 feet away from the main panel, on the same inside wall as the main breaker??

Yes, you would just need an available space for a standard double wide breaker in your main breaker box. I used a 30 amp Square D breaker in my main breaker panel from VD that had been cryo'ed. I can run 4-5 outlets from this 30 amp breaker.

If you can do it this way, you would avoid the need for any exterior rated wires, panels, etc. I'm assuming you could then run ded. lines in the wall from the sub-main to your individual outlets. Craig

Disclaimer: I sell the stuff cause I like it so much!
I DO find that using all-Teflon dielectric for dedicated lines DOES result in a cleaner top octave and absence of low-level noise or "phasey" artifacts from dielectric effects from cheap insulation like THHN or Romex.
At $2300/1000' spool Belden's 83803 12AWG/3 is hideously expensive for commercial/industrial/household use, but at $3.50/ft for audiophool use it's a GREAT tweak.
Certainly establishing a dedicated line of almost ANY type is more important, but if you can do it in Teflon for only a hundred or two you WILL hear the improvement. Cheers.

Thanks for all the great tips, advice and input guys!

BTW Subaruguru, where can you purchase the Belden cable?