Power cords - a BASIC question

I have found that what I perceive as the conditioning effect of a good power cord can become too much in a long length. There definitely is value in using very thick guage and high purity copper in the wall, and other stuff that has been discussed elsewhere, but when I once tried running very long runs of high-end PC from the switch board to my amp the sound was not good.

It's a basic question that needs a basic answer. Start with going to the powersnakes website (www.powersnakes.com) and reading the faq's section. A common assumption is that the power cord is at the end of the circuit. This is not true. Power runs from hot to neutral through the load. The load is the component. The power cord is in the middle of the circuit, with the component in the very middle.
Power cords do not change sound. They provide current. If they also filter out RFI and EMI, they will give a "blacker" background to the musical signal. Thus the music can change. Each system and locality may have differing quantities of RFI/EMI thus affecting the music differently.
All RFI/EMI must be eliminated for the effect to be at it's most effective. The RFI/EMI around the wires in the walls should be small enough and is not near to IC's and speaker wires to have an effect. Contact areas between power cords and IC's and speaker wires need to be kept to a minimum.
BASICS.

One comment to make is that ALL equipment is 'voiced' to have a certain sound. changing the incoming power (from a wide/rough sinewave to a thin perfect one) may drastically alter the possible sound to a degree that it becomes "bad" My take on this is the "thinness" that comes from too much power putzing.

Systems with digital components are more sensitive to power cables than purely analog systems. Digital components are notorious RF producers and can easily pollute the rest of a system. The filtering in power cords is probably doing more grunge suppression on the ground from the digital component than on the incoming electric power.

In order for a line cord to alter the signal that passes thru it, it would need to have in it the things that are used to alter signals. That is, it would need a filter network of some kind. Filter networds generally consist of capacitors, resistors and inductors. Line cords are made of wire and insulating material only. Also, keep in mind that the internal circuitry of your equipement does not use electricity in the form that it is delivered from the wall to the equipment's power supply. The power supply brutally hacks up the incoming sinewave and reconstitutes it, so to speak, before it is use by the internal circuity. The process itself that the power supply uses to ready the electricity for use is generally far more trouble (RFI) than junk that may be on the house wiring.

Your point about the wiring in the wall is valid. Electricity moves around in a lockstep fasion. If there were such a thing as a good wire and a bad wire and you put these wires in series (a line cord is in series with the household wiring), whatever was happening in the bad wire would also be happening in the good wire.

If you imagine electricity as water in pipes, the pipes have to be full. So, when pipes are hooked together water that moves in one has to moved exactly the same in the other.