The text recovery is quite noticeable, but I was unable to remove enough background noise without loosing some text outline, even with intermediate processing in a specialized book restoration package before extra processing in DjVU cleaner & converter. In other words, we’re facing the same task of narrowing the text outline all pixels color spectrum closer to black color before binarization.
Besides, this RawTherapee TIFF format can’t be converted directly by DjVU tools for some reason, thus requiring be re-saved as TIFF by another graphical package.