I have had two very different experiences with power cables.
First:
I bought a pair of Cardas Clear PCs sometime back and put them into my Coincident 300b mono blocks. The difference was night and day better. It was stunning, I became a believer that a PC is the most important cable in the system.
Second: Same PCs
After upgrading from Coincident 300b mono blocks to a Nagra 300b stereo amplifier, there was no difference between using a Cardas PC and a stock PC. It was a big shock.
Third (bonus experience):
I tried the Cardas PC with my recently acquired Shindo preamp. It sounded worse.
Shindo pre/power are famous for only accepting their own cable that looks like an ordinary stock power cable and is free.
My understanding now is that it is very system-dependent, and better the quality of your amp/preamp the smaller is the effect of a PC and vice-versa.
Now finally, here's a shocker.
I have a power strip/passive power conditioner made by Acoustic Revive.
Based on my second and third experience, I decided to get rid of it and removed it from the system to photograph and sell it, and suddenly my system was unlistenable. The harmony was gone, the instruments were suddenly playing independent of each other and not together with the music. I put the AR power strip back into the system and boom! it all came back together again, it was again so surprising that I called my non-audiophile GF to listen and even to her it was very clear that the AR power strip was doing something special.
The AR power strip has no active electrical/electronic parts!!