Do not increase the wire size beyond 12 ga. as this is a code violation

Not true...

FWIW:

National Electrical Code 90.1 Purpose:

(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity. This Code is not intended as a design specification or an instruction manual for untrained persons.

(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance result in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

NEC could care less how your audio system sounds. It could care less if you have ground loop hum. No where in the code is it mandated voltage drop shall be a consideration . NEC is bare minimum electrical safety standards.

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12 gauge non-dedicated line

A non-dedicated line is [almost] always a bad idea. Refrigerators, hair-dryers, SMPS, light dimmers, etc. add nasty stuff that some products cannot tolerate.

When we built recording studios, the gear and the garbage were often on separate services to the street! One well-heeled owner installed 100% system isolation via battery. IIRC, Sony NYC had its own gen set in the basement.

A non-dedicated line is [almost] always a bad idea. 

I concur

I just completed my 50 foot dedicated AC.

Your question was what wire to use.

I believe I addressed this issue with about the best wire available.

Oyaide high quality copper three wire 10 gauge.

This very special cable is available through the US distributor, www,lotusgroupusa dot com. Joe Cohen in NorCal. 
This cable is very close to a good quality power cord. Its construction includes 2 copper grounds in addition to the main three wires. 1 a ribbon that wraps around the main 3 and a second one in a thin copper braided wire.

To get the best, one has to pay. My 50 feet cost $4K. In addition I terminated the run with two Oyaide duplexes and carbon fiber face plates. This was $1200 more.

And my electrician was another $2200. My run was complex through the attic.

Was $7400 worth it? You bet! The improvement was global. I got about a 25% better system. Clean, punchy, stunningly real!

The only catch is that none of this Oyaide stuff is UL approved. But the Japanese standard are way beyond those in the States. And I did not touch the house AC system. Still, you have to find a licensed electrician who is willing to deal with non UL. I went through a couple and luckily found a guy willing to do the install.

And I had to pay him a consultant fee to talk with Joe Cohen on how to deal with the unique Oyaide cable.

The main issue is wire. For me, there is only one.

It would have been really sweet to run a parallel 10ga Romex and charge for  tickets to demos of the improvement...