Catalyzes the reduction of free flavins (FMN or FAD) by NADH. Subsequently, the reduced flavins diffuse to the HsaA oxygenase subunit (By similarity). a reduced flavin + NAD(+) = an oxidized flavin + 2 H(+) + NADH Lipid metabolism; steroid biosynthesis. HsaAB monooxygenase consists of an oxygenase component HsaA and a reductase component HsaB. Belongs to the non-flavoprotein flavin reductase family.