This might be a good time to chime in to report that I removed the PP from my system a few weeks ago. I had suspected for a long time that the PP was injecting noise into the system and wanted to remove it to test its effect (or lack thereof in that case).
It turns out that with the PP in my system, micro dynamics are killed and the music flows less and is less relaxing.
With the PP in the system, it may seem that macro dynamics are improved. But I believe it is more a question of noise being added, which is perceived as additional volume. Yet since details are lost in noise, I don't think the overall dynamic range is actually improved...
It took some time to do some A/B comparisons, several times, over several sessions/weeks to be absolutely sure.
I know that the PP never truly isolated mains power as I could always here whenever other appliances were plugged in the house. But I had left it in as I thought dynamics were improved somehow. But now I know what the PP takes away, and I don't want to lose that.
In the early days I was very happy with the PP running on batteries only. But I had only around 45 minutes of life when operating from battery, and this dropped as batteries aged. So I almost never used it. And I don't know if on battery, it was killing micro details or not...
So at the moment the PP is back in its box.
And yes it may be that I have a faulty unit, but I am not willing to go through hoops to (maybe) make it work. I sent the unit to the factory several years ago and getting it back was not easy.
And no I do not believe this is a ground loop issue (unless the PP internal design itself adds such a loop) since I have no noise at all currently, and the only difference is that the PP, which was inserted before the audio system into the same socket, is now removed.
Like all devices the PP adds noise (and probably more than most considering what it is), and I am better off without it (no garbage in, no garbage out).
So yes as Paul said, power devices "do not just quiet the AC but they affect the sound of the hi-fi".