High quality in-wall UL-C2 rated 10-gauge A/C wiring ?

As far as the 2x14awg romex test was concerned:
1. Take two 50 foot lengths of 14awg romex.
2. Take the two black hot leads of both cables and connect them to the same 15A/20A circuit breaker.
3. Take the two white neutral leads and connect to neutral bar in sub-panel.
4. Take the two bare ground leads and connect to ground bar in sub-panel.
5. Run the two 14awg romex cables to outlet.
6. Connect the two black hot leads to hot connection on outlet.
7. Connect the two white neutral leads to neutral connection on outlet.
8. Connect the two bare wire ground to outlet ground.

So, essentially, I am doubling the size of the wiring from circuit breaker to outlet. This is the same thing that happens inside larger power cords.

auxinput

1. Take two 50 foot lengths of 14awg romex.
2. Take the two black hot leads of both cables and connect them to the same 15A/20A circuit breaker.

You'd need to check NEC on this as well as your local code. Also, note that only breakers designed for such a two-wire connection would be permitted.

This is the same thing that happens inside larger power cords.

No it's not.

Even if R were the same, L&C are vastly different.

In the 'test' did you also have the heavier ga single wire connected to the same side of the transformer and routed closely so you could listen to each with minimal delay? If not, there innumerable changes possible on the A/C

Yup, I totally know that it's not to code.  My point was that 14awg is really not enough to supply good current to audio equipment.  The on-demand current draws are just restricted by the small 14awg wire along with the high dielectric insulation (this is not anything like foamed polyethelyne or teflon found on many good power cords).  While ieales seems to think that 14awg is perfectly okay for 20 amps of current (and it may be for general purposes, but highly not recommended!!!), is it definitely not acceptable for the fast current draws required by audio equipment for good sound.  You guys can laugh at me all you want, I really don't care because I know what works and what doesn't.

Headroom loss for 14ga vs 10ga is less than ¼db @ ≈14A / ≈1600W
See http://ielogical.com/Audio/CableSnakeOil.php#ACWiring