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

Do you have a top quality power cord to connect the P15 to your home service?

If it’s a demo unit, it probably has already been broken in unless the dealer told you it wasn’t on the floor very long.
If it’s that far off, call ps audio about your issue. It should be under warranty and they are very good at supporting their customers 

Could be  the Pass amp just doesn’t like any power conditioners with active devices. 

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?