Catalyzes the reduction of the macrocycle of precorrin-6X into precorrin-6Y. NADP(+) + precorrin-6B = 2 H(+) + NADPH + precorrin-6A Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 6/10. Belongs to the precorrin-6x reductase family.