Mediates C-methylation at the 8-position of the aminocoumarin moieties in coumermycin A1 in the biosynthetic pathway of coumermycin antibiotic. Active on both mono- and bis-amides for mono- and di-C-methylation adjacent to the phenolic hydroxyl before it is glycosylated by CouM. kcat is 2.2 min(-1) with desmethyl-monoamide as substrate. kcat is 1.23 min(-1) with monomethyl-diamide as substrate. kcat is 1.38 min(-1) with monomethyl-diamide as substrate. kcat is 1.71 min(-1) with desmethyl-diamide as substrate. kcat is 0.23 min(-1) with desmethyl-novobiocic acid as substrate. Antibiotic biosynthesis. Belongs to the methyltransferase superfamily.