Every musician is entitled to an off night now and then; Saturday was one of Maria Muldaur's. Unfortunately, it was not a night off - she and the David Bromberg Bank co-Theater.

For the sake of democracy (or to avoid changing the set an extra time), the booking order was Bromberg-Muldaur the first show, and Muldaur-Bromberg the second. Undoubtedly it worked better the second time, since Bromberg wound the audience up to a pitch his co-star could not match.

Bromberg shares with the Nitty Gritty Dirt Band a talent for combining country, jug-band and washboard, bluegrass, swing and rock into an irresistible mix. He also shares with the Dirt Band an amazing flexibility; among the instruments wielded by his seven-member band are fiddles (sometimes three at once), banjo, mandolin, guitar (Bromberg flat-picks to beat all), trombone, flute and saxes of all sizes.

The chillingly clear soprano which has been Muldaur's national trademark since "Midnight at the Oasis" was strained and cloudy Saturday. The problem was most noticeable on "Oasis" and on Dolly Parton's "Tennessee Mountain Home," where the ear filled in Parton's own soprano. The lower blues and swing numbers, however, for instance, Jimmie Rodgers' "Any Old Time," sounded full and rich.