I agree that a lot of power cords are not efficient because they do not work to cure the pb of EMI and RFI activated by the power cord itself. I work with many manufacturers of audio gears (very famous) that do not believe in power cords. I made some demonstrations with on of them, may be the most reluctant about PC action. After a long explication why PC could not work (He is Engineer in electronic and designer of probably the best amplifiers), I (engineer too but mainly in acoustic and mechanical vibrations, formerly professor in a University) made a test : blind listening with standard power cord of his famous amplifier and then with a highly regarded power cord that ASR say he could not work. All the people in the room said the second listening was vastly better! collective hallucination? electric compensation of ...? In fact I know why it's better and yes I agree 90% of PC change the sound but not improve it, but some do. I also made tests (blind) with audiophiles and same result. I conducted finally the same test with skilled musicians, same result. Some PC improve music playback and not only change it for a different taste.
The conclusion: as a scientist I always try to find why and how a phenomenon occurs.
This is the same subject that power conditioners. 95% do not work well because they wrongly "depurate" 230V. 230V should be perfect sine wave with no deviation with no harmonics... even if some electronic power supply make a very good job to filter AC at the input and they store energy. Only hearing transformers "singing" without music because there is some DC or too much pollution in AC that the power supply could not treat enough. This time too, proof is in the listening.
Scientist approach is mandatory but it should be well done and not conclude to wrong judgments due to inappropriate measurements (I spent 10 years making measurements).