I'm neither a skeptic nor a believer in power cables. I'm listening to my system that has about $3K in after market power cables from several different manufacturers simply because I don't want to not have the best interface to power I can afford. But, I never hear any difference in my system regardless of whether I'm using the equipment manufacturer's power cord or the after market power cord.
One of the problems with comparing before and after effects is the fact that you're relying on your memory for the base line and then attempting to find the difference in what you're now hearing. Not a very good method of analysis.
One of the problems with using test equipment to measure differences is that you setup the equipment to find the differences you THINK you might find. Instrumenting a test setup to find "everything" is difficult as you don't know what you're not looking for.
As an example, I had a custom built, image processing system in a VME rack and it was not working properly. I traced the problem to the VME rack power supply. I removed the power supply and took it in to the electronic engineers to be diagnosed. They found nothing wrong with the power supply - correct voltage, correct amperage delivery, stability OK, etc. So - they said "Nothing wrong with it."
I knew there had to be something wrong with it and went in to talk with software engineer who was also an electrical engineer. He said, "I'll take a look at it." About two hours later he said, "I've found the problem." "There is a 250 KHz signal on the power." "That signal is having a detrimental effect on one of the chip clocks and disrupting its processing."
So there you have it. The EE's who originally looked at it had a fixed set of power supply problems in mind and verified their bias through testing for only what THEY thought could be a problem. The person who found the problem decided to look at the QUALITY of the electrical output and used a completely different test method and equipment.
Then you get back to power cords, interconnects, and speaker cable - you have no "test instruments" other than your ears and memory - and you want to hear a difference.
I can hear differences in speaker types and designs, and can and have heard changes in my system when I change out certain pieces of equipment. But cabling and power cords...I can't or don't hear any difference.
I don't hear a difference, but I'm willing to admit it may be my insensitivity to the change. So, I continue to make purchases of power cables and interconnecting cables with the hopes that I too will be "blown away" by the difference....only, so far, that's never happened.
Some people do hear a difference and that's okay. If it improves their listening experience that's really all that counts and makes the investment in the cables worthwhile.