I have done a lot of experimentation on this and find that a very large percentage of sound changes due to power cords are the effect the cords have on the transformer resonances in the box.
Almost all transformers have fairly high Q resonances in the ultrasonic area, right where switching supplies and silicon diodes emit noise, thus stimulating these resonances. This ultrasonic noise can wreak havoc in many pieces of equipment.
The power cords are an important part in determining the resonance frequency and Q of these resonances.
Not only does the result of these resonances cause problems in the device the cord is plugged into, but it can also travel through the cord into other boxes, stimulating THEIR transformer resonances. (this is not wild fancy, I have measured this many times)
The net result is that a power cord CAN have a significant affect on the sound of a stereo system. The problem is in correlating it to anything. Since each system is different with different combinations of transformers its very difficult to make recommendations as to which cord design is going to be "better" than another. An expensive cord may very well sound better than a cheap one, but it may also sound worse.
The best way to deal with this is to damp the transformer resonances in the boxes themselves, its not hard to do, but hardly any manufacturers DO it. I think primarily because nobody knows its a problem. Its a significant affect that for some reason seems to have completely flown under the radar of the audio design community.
So you can either damp the transformers in all your boxes, OR spend time playing with different power cords until you find a combination that reduces the resonance affects.
BTW these resonances are electrical, NOT mechanical, rubber washers and the like isolate mechanical resonances not electrical ones.
