Either/Orchestra, Saturday, October 15, at the Bluebird Theater, is the odd, eccentric, precocious (but dearly loved) brainchild of composer/ saxophonist Russ Gershon. And a hardy youngster it is, for the Boston-based ten-piece jazz ensemble is now celebrating its ninth year of life. This longevity is quite an achievement: Aside from the combos led by Duke Ellington and Sun Ra, very few large groups have had E/O's staying power or produced such a unique and identifiable sound. A cross between Kesey's Merry Pranksters and the Modern Jazz Quartet, the Orchestra--featuring a drummer, a bassist, a keyboard player, three saxophonists, two trombonists and two trumpeters--has made its reputation by bending standard jazz formats to fit its players' bizarre worldview. The Brunt, a followup to the act's Grammy-nominated Calculus of Pleasure album, is a case in point: The disc features vibrant original music as well as a cover of Mal Waldron's classic "Hard Talk" and a very cool, bopping version of Bob Dylan's "Lay Lady Lay." The band's humorous. It's creative. It's alternative jazz--if there is such a thing.