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.
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.