My location with its AC quality and my system seem to be very prone to AC noise degradation. I find that the AC iPurifier makes a profound difference on my system. This is with HiFiMan HE1000V2 headphones, Woo Audio WA6SE amp, Yggdrasil DAC and Ayon transport. The noise floor lowers considerably, layers of grit and grunge are removed, the overall sound is much more transparent.
I was already using a couple of homemade passive plug-in AC filters, simply a couple of small value polystyrene capacitors (.0056uF 600V) connected across AC and Neutral. By themselves they caused a distinct though smaller improvement along the same lines as the AC iPurifier. I found that with these small value filter caps that presumably affect only very high frequency noise on AC, the capacitor dielectric is still important; it still seems to affect the sound quality. Polystyrene is the highest quality inexpensive dielectric used in capacitors available in these values.
The iPurifier greatly magnified the improvement.
As an experiment, after plugging in the AC iPurifier I removed the homemade AC filters. I discovered that these small value polystyrene caps were significantly enhancing the effect of the iPurifier. They are essentially an effective tweak to the iPurifier. I theorize that they introduce a better dielectric (polystyrene versus probably polypropylene or polyester or perhaps even ceramic used in the iPurifier) to the passive filter in the iPurifier, and perhaps slightly extend the frequency range of its coverage.