There are 2 problems :
1) I'm joining 2 sources together - one (SE DVD - sourced) has black bars really black (0, 0, 0), the second one (GOUT upsized to anamorphic and color corrected) has black bars (16, 16, 16)
2) I have done color correction of this problematic source, and the only way how the colors look "natural" is with the 0-255 "range". In other words, the avs script looks great while playing it, but the colors of the resulting mpv file look pretty bad