With all the difficulties that I've had with my display, trust me when I say, do NOT, under any circumstance, reset your display. Doing so throws off the panel's aging algorithms, necessitating voltage adjustments to remove resulting artifacts including, but not limited to, magenta misfires, black rain and image retention. I was not experiencing any of these prior to reset, and achieved inky blacks by reducing only RSTP and S1, S3 and S4 (the latter 3 by the same amount) - while staying within the factory recommended ranges.
After reset, VOL SUS needs to be increased to remove magenta misfires, which are a symptom of under voltage. Despite my past observations, RSTP needs to be increased in order to remove black lag / black rain. I had to increase mine to 36 to have absolutely no sign of black rain. RSTP seems to be the voltage that "wakes up" the sleeping pixels, and after resetting, this voltage (more accurately, the value of RSTP) needs to increase. Prior to reset, I could set RSTP to 2 with no sign of black rain. The problem with raising RSTP after reset is that black level suffers a tremendous increase. To counter this black level rise, I reduced S1 and S3 to minimum, while keeping S4 at factory default (to avoid the 16 it color effect that I have observed when setting this below default), which gave me inky blacks at a cost - black rain is back (though not as aggressive as when lowering RSTP). Only way to remove black rain at this stage was to increase RSTP to a whopping 60! Though, as you might guess, my blacks went back to their level when I had RSTP at 36 and S1 and S3 at default.
What I ended up doing is, I set RSTP to 0 and turned off the display (pressing the power button on the bottom left). I turned it back on and RSTP reverted back to its factory default (18). From what I understand, this is the only voltage that is set at the factory per display, so my factory default will not necessarily equal yours. Once RSTP was at default, I increased VOL SUS to 168 (this is the value that cleared magenta sparkles on all internal patterns) and reduced S1 and S3 to factory minimum to lower black level. I reset the display and will proceed with running slides so the panel settles in.
Pioneer Kuro KRP-500M
Pioneer Kuro PRO-141FD
Pioneer Kuro PRO-111FD x 2
Pioneer Kuro PDP-5020 x 2