I got a KRP-600A (Nov/Dec 2008, 19.500 hours) and ended up with RSTP at 12 and SAD at 108 after a year of experimenting with weirdest parameter values. In combination with LED backlight I got nice blacks and no artifacts during motion picture. Only some magenta misfires when black bars remove for commercial breaks.
Magenta misfires commercial break:
Bought from first owner in February 2019 with 17.400 hours. First owner used it from June 2009 to January 2019.
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First test in darkened living room with daylight falling in. It looked like it was free from red tint. Happy and hoping!
Original parameter levels after 17.400 hours and 9 1/2 years in use.
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Pitch black apartment rooms revealed the notorious red tint. Damn!
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Extreme tweaking achieved nice blacks...
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...but also heavy artifacts.
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As mentioned I ended up with original adjustment, only RSTP/SAD tweaked to 12/108 and LED multicolor backlight (dark blue seems best for blacks).
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With backlight on, blacks are very good. New OLED TVs might be better and blacker but I'm quite satisfied with my old Kuro and enjoy the natural picture quality in Optimum Mode.
If you have any ideas for more perfect tweaking without magenta misfires after removing black bars, let me know. Setting RSTP at 1 caused nice blacks but also magenta misfires during motion picture, even though it was rarely.