Hi Sjh32,
A better power cord indeed closes the gap a little between the X1 and X2.5, only if the X1 is using a stock cord. Obviously the playing field would not be so level and more so if you put a better cord on the X1. I have tried the PS Audio P300, and the UO and have found the difference to be more than subtle. One thing to make very clear and that is the X1 is a superb pre-amp by any standards- it is indeed a reference amp. Compared to the X2.5, the X1 has a lower noise level, more details and a better grip on dynamics (in other words when musical passages gets a little more complex, it allows the music to expand even more naturally).
However the essence with all Pass amps are still the same which is the same world famous Pass Labs "neutrality" in the pre-amp and a fantastic soundstage with strong bass.
I have compared the Pass Labs X2.5 to every Classe pre-amp model except the Omega and it has trumped them all including the CP-65. Even the PS Audio PCA-2 (a great value), Linar Audio pre-amps, Copland pre-amps cannot measure to the X2.5. No pre-amplifier is completely neutral but the X2.5 has less coloration than all the other amps I have tested. With the $2,000 difference between the X1 and X2.5, one can actually buy a brand-new P300 with more than enough money left to spare for a couple of reference power cords. Of course, if you can afford the X1- go for it because it is quite good. But like everything else, when you end up buying power conditioners and cords, you will be spending another $4,000 more for a difference of less than 10% and sometimes 5%.
Your thinking of having the P300 makes sense because it can help clean up the AC in your system and thus does not just benefit the pre-amplifier alone. At the end of the day, you may just have to do a listening test to determine if this is the amp for you.
Hope this helps. Feel free to let me know how things go in your search.
Zenaissance