The adult male art is really old and just looks darker or oddly shaded when compared to female poses or custom lineart muties. It makes markings look so dingy compared to the other poses. The lineart also looks thicker and darker in some areas like the older mane types and the faces are kinda dull and expressionless, which might have been intentional.

The suggestion is just to revisit the files for adult male default lineart and clean it up a little bit to bring it more in line with the newer art. No redrawing or redesigning should make this an easier fix than new male poses (still waiting in anticipation for those).