Among the finest large jazz ensembles -- not only locally, but nationally -- the Jazz Unit has been playing regularly on Monday nights at the Bop Stop since 1994. But since that venue is temporarily closed, the band is doing this gig at Nighttown to keep a presence on the local scene. The Unit has already cut one CD, Choices -- an all-around excellent effort that features impressive compositions and arrangements by bassist Dave Morgan. Some of the finest jazz players in Northeast Ohio appear on it, including trumpeter/leader Jack Schantz, alto man Howie Smith, pianist Dan Wall, guitarist Bob Fraser, and vibe player Ron Busch. The Jazz Unit contains 13 pieces, including two trumpets; two trombones; a French horn; alto, tenor, and baritone saxophones (the tenor man doubles on flute); guitar; vibes; piano; bass; and drums -- an unusual instrumental makeup that leads to fresh and unique timbres. Morgan's got a modern, mainstream writing style that's been influenced by Bob Brookmeyer and Thad Jones, and the band also plays charts by Smith, a member of Cleveland State's faculty. Among his compositions are the 7/4 "In the Kitchen" and "Wayne's Whorl," a Wayne Shorter-like piece. Schantz has always been an inventive and melodic improviser, and his concentration on classical playing in recent years has given him a fuller, darker tone, better range, and more precise articulation, while Fraser has been influenced by pianists Bill Evans and Chick Corea, and he's a very intelligent improviser, as is Busch. John Clayman's a strong, lyrical tenor sax man, and Morgan also solos well; he and drummer Mark Gonder give the band a powerful, flexible underpinning.