PS Audio Power Plant 15........

The final word on any piece of gear, of course, is how it sounds TO YOU. Don't convince yourself something must sound better simply because of price or name recognition. That said, I'm a regular watcher of hi-fi reviewer Steve Guttenberg's YouTube channel, and I remember being startled when he said that he tried a PS Audio regenerator once, and it clearly, without question, worsened the sound quality. (Who knows why?) He ended up getting rid of it. Guttenberg is a huge Pass fan, and has a Pass amp in his current system, though I don't know if he had one when he tried the regenerator. I also remember stumbling across a comment on a hi-fi forum (maybe this one) in which someone said that Nelson Pass once said that using Pass amps with other power components (he might have specifically referenced PS Audio regenerators) is often not a good idea, because the power section in Pass amps is already so good that most other power components will be inevitably be a downgrade. That might not be the exact way it was put, but it was something very close.

How does clean power worsen the sound?

That’s what happened in my system as well....ruined all nuance, air, 3 dimensionality, etc. All systems are different...in mine not good. (All Ayre)

I purchased BHK preamp, M1200, and P15 together three weeks ago. The first week they sounded flat and dull, but now they are phenomenal. It may well be PS Audio designed their gears to match well with their power plants. P15 may not give as much improvement to other brands as to their own.

Another huge improvement was adding an Audioquest Thunder AC cord to P15. It made a dramatic improvement too.

I think “huge” is an overstatement. I had a Pass Int250 with a dCS and in my opinion there were some subtle improvements - PS Audio gives you a more rounded sound and a blacker background but I can understand why in some systems the leaner sound  without a PP would be preferred / work better.