I have three aftermarket power cords in my system, two are by
VH Audio and the other is from Transparant. The Transparant came with my Arcam CD player. The owner of the store I bought the Arcam from really tried to get me to buy one, saying it would significantly improve the sound. When I balked (I didn't believe it would make any difference), he pulled out one of his demo cords and gave it to me gratis, saying it WOULD be important. I never A/B'd the cords as I still was skeptical. Don't even know where the original cord is now.
Fast forward to about a year ago. I was moderator on an audio board and there was a thread involving several board members switching out the cords on their Rotel preamp/processors with one from VH Audio, with very positive results. I kept on questioning whether they could be right and they kept on urging me to give the thing a try. I did so, still doubting, but at least willing to find out for myself. They arrived. I set my system so that I could easily A/B the things. To my amazement, the sonic benefits were undeniable. The ground floor was significantly reduced. Sonically, the system seemed to sparkle. I couldn't figure out why this should be so I posted over at AudioAsylum. They speculated that it wasn't the grungy electrical power coming into the house, but, due to better shielding, electro-magnetic fields radiating from things like my amp weren't affecting the Rotel as much.
I don't know what the real deal is, but I do feel very strongly that the cord made a difference with the Rotel. When I bought one for my Denon 1600 DVD player, I was unable to detect any difference. When I brought this hit and miss thing up with a manufacturer of cords, the response was that this is to be expected, depending on the component, the positioning of other cables, the quality of the cable being replaced, and other factors such as this.