OP You installed the upgraded power cable on your CD player and you heard an improvement in SQ. You did not say you "I think I heard" or "its my opinion that it sounded better". It sounded better. Period.
Yeah, it wasn’t subtle at all and I wasn’t expecting it either. I think sometimes as audiophiles we can get all excited over a new component and sometimes hear things that aren’t there because we expect there to be an improvement. This was not one of those times.
Don’t let anyone push you off the fact that you can hear a change in your system.
Don’t worry about that, I’m too far down the rabbit hole. Save yourself! After hearing the change in sound when I swapped from stranded core to solid core speaker cables, my curiosity was peaked. After that was experiencing differences in RCA cables. Then power cords. I remember when I first heard these differences, I had mixed emotions. I was happy to learn that you can tweak sound with cables but saddened because I knew if I wanted to get the most out of my system I would have to spend a lot of time (and some money) to figure out what worked best in my system. In hindsight it has been a fun, rewarding pursuit.
I don’t think power cables should be as mysterious as they are. There are a lot of good suggestions in this thread, but I have yet to see a published scientific study covering any of these ideas in depth. I agree with nonoise that it’s a possibility we just don’t yet know how or what to measure to understand certain complex phenomena in the world of audio reproduction.