The original C6 electrolytic capacitor was rated at 100uF/6V in the PL700B (there was no C6 in the PL700). The early PL400 boards (PL 400C/PL14A) used a 100uF/10V capacitor for C6, later PL400 boards (PL14B) changed C6 to 100uF/6.3V because the voltage across the device should never exceed a few volts.
In the initial (PL14-PL20) White Oak Audio boards the design for C6 called for a 100uF/100V device. For the revised White Oak Audio boards, the rating of C6 was changed to 470uF/25V to improve low frequency response according to the designer of the board.
The consensus is that a 470uF/25V capacitor in the C6 position for all the PL amps is an improvement, the 25V rating is overkill but the combination of 470uF and 25V is a common design.
I tried to tell Lee back in 2011 that the Blackgate capacitors in C6 made my PL700B growl, but he clung to his Nichicon Muse capacitors (actually, very fine devices). Now, I believe he is a convert.
Mark