So no wonder that in test after test, we find no difference in quality of AC mains, or output of audio device tested.
Why do people attribute better fidelity to these devices? Simple: lay intuition says "filtering" removes noise. So they pay more attention to what is playing and "hear more into the music" with the AC conditioner in the path. And report veils removed, noise floor going down, blacker backgrounds, etc. This happens even if I gave them an empty box but told them it is a power conditioner.