What is the rest of your system ...source, interconnects, speakers, speaker cables?
Do you use a power conditioner, power strip, or plugged directly into wall?
I had several Audio Research tube preamplifiers throughout the years that I used to drive the Pass Labs X250.5. Even though an impedance ratio between those preamps and the X250.5 was not ideal, I never ever experienced brittle high frequencies. The power cords that worked and still work best in my system, to my ears, were Shunyata higher end cords. I used Taipan and Python cords, and the results were always superb. My recommendation would be to give the new caps at least 100hrs of playing time before making any changes.
For cables, try borrowing few power cords from cable company, it’s worth a try to determine what works best.