I agree with KF...
I have used PS Audio with very good results on less expensive gear (eg. Sunfire Amp, Theta Miles).
Improvement in the Theta was nice but the sunfire was a different story...
It was like I had bought and brand new amp. it was incredible..
In my opinion, the sonic improvements gained by a regenerator such as the PS Audio are proportional to the quality of your incoming ac and the power supply(s) in your gear.
When I upgraded to very good amps (Atma Sphere MA1 mkII.2) the improvements were positive, but far less dramatic than the sunfire.
I still use a PS300 for front end.
Although, I was most suprised by the installation of dedicated balanced ac lines with good receptacles?
I have heard great results with these and it was somewhat less expensive than the very big and good generators/conditioners.
I have not heard the other choices but have read great reviews on Hydra.