Catalysis of the reaction: (6R)-2-acetyl-6-(3-acetyl-2,4,6-trihydroxy-5-methylphenyl)-3-hydroxy-6-methylcyclohexa-2,4-dien-1-one + NAD+ = (S)-usnate + 2 H+ + NADH.
reduced-(S)-usnate:NAD+ oxidoreductase (ether-bond-forming)