Why Expensive power Cables when Romex behind Wall?

In my own experiments, the introduction of 2 feet of stranded 16/3 extention cord onto the end of my preamp, turntable or power amp was enough to influence the sound negatively. Keep in mind, the typical 18 awg cord supplied on equipment, has a up to 7 amp capacity. Resistance played no part in the additional 2 feet. If your speakers have at least the resolving power of an LS3/5A, anyone with an open mind should be able to hear the effects.
A simple tool called an Elfix, can eaisily demonstrate the incredible amount of stray currents surrounding A/C wire.
Shielding almost always results in degredation. The ultimate dielectric is AIR, followed by glass. Since these are all near impossible to achieve in a wire, we are stuck with compromises like teflon.
As usual, everything Albert states is true. I tend to agree with BWhite as well. My guess is the wallplates will most definitely hurt the sound if they are metal.......Frank

The reason is simple: They sound better. Have you listened to a better power cord yet?

Audioengr,
To defend my honor I must clarify my statement about shielding,
"Shielding the cables as they near this big rack of electronics may make all the difference. Perhaps the worst dirt is the dirt generated by your own system feeding back into itself. This could explain why shielding may help."
I'm not talking about mechanical resonance, I'm talking about EMI from equipment feeding back into the system through the power cord. This sort of phenomena would happen if you placed unshielded interconnects right next to your speaker wire. The output signal would then be fed back into the amp in some form.
That being said, I do not make any claim how this could affect the system. I'm simply offering way that I think things could happen. I did say I know just enough to be dangerous :)

Albertporter wrote: "I have Soundlab Ultimate One speakers, and the brands of AC cables connected to them and to my (home theatre use only ) sub woofers make a huge difference in sound. Crazier still, disconnect the sub woofers from the wall and the performance of the Soundlab is improved. The amplifier and resulting magnetic field of the sub woofer ARE interacting with my Soundlab."

This is certainly possible. Sounds like you have a combination of magnetic field coupling and mechanical coupling from the sub. Subs put out a lot of energy that can certainly resonate the shutters and probably some component chassis. Power cords make a difference for reasons other than mechanical resonance or magnetic coupling. However, if the subs are on the same circuit as your other components, they may be causing droops in the AC line that the other components are reacting to. Sounds like a complicated problem at best. There is no way it can be diagnosed over the internet.

The thing I want to point out is that it is easy to get sidetracked from root-causing the real problem, particularly if the physics is not well understood. I have seen posters change interconnect cables over and over to solve a ground-loop problem, when the real problem is the AC power grounding.

Frap - I believe that resistance played a part in the added 2 feet, and inductance as well. The 16 gauge cable 2 feet long can have the resistance of 20 feet or more of 14 AWG ROMEX. It has the inductance of about 12 feet of ROMEX. So this was like adding 20 more feet of ROMEX to the circuit.

I agree with you on the air dielectric being the best, however the goal with a power cord is low inductance and resistance. This means large conductors or lots of smaller conductors that are tightly magnetically coupled. Some cords do not do this coupling well and the result is excessive stray magnetic fields. Power cords do not "pick-up" stray fields, but they do create them. If you have unshielded interconnects running side-by-side with a power cord to an amp, you might pick-up some magnetic crosstalk in the interconnect. Incidently, shielding a power cord is a bad idea, IMO, because it adds unnecessary capacitance. Typical shielding also will not stop the stray magnetic fields for the most part. You need MU metal shielding to do this.