The P-300 is current limiting and should not be used with any power amp, not even a low-powered amp (in my opinion, no PS Audio regenerator, even the big one, should be used with any power amp other than a very low-powered tube amp, as they are current limiting on peaks).
The way the PS Audio equipment is generally run is the Ultimate Outlets, which do not limit current on consumer-type power amps, on amps, and the power regenerators such as the P-300 on front-end equipment. While you can generally "add up the wattage" of your front-end equipment and, if within the P-300's wattage rating, run everything successfully from a P-300, certain front-end equipment can overload them on start-up (BAT 51 SE preamp) and will require the bigger models.
The more important question is what these components will do to the sound, as they all affect the sound, especially, as a general rule, the P-300, which in later iterations can vary the frequency of power and "force-feed" the power supplies of front-end equipment it is running. My experience of running a P-300 with my front-end equipment and a pair of 20-Amp Ultimate Outlets on my monoblocks for over three years has been extremely positive, but especially for those with very high-end systems, the affect of these devices can be dramatic -- they should be demo'd in such systems before purchase.
