The tuba, the basso profundo among brass instruments, is handicapped by a straight-and-narrow range and a booming sonority, yet unwieldy as it may seem, it has its ardent practitioners and fans--including the thousands who will participate in next week's International Tuba/Euphonium Conference at Northwestern University. One aficionado is John Williams, who likes to garnish his movie scores with tuba wails. Ten years ago, to mark the centennial of the Boston Pops, which he then headed, Williams even wrote a Tuba Concerto, a quasi-Wagnerian romp that turns the tuba into an unlikely hero with engaging braggadocio. This seldom-performed concerto will be debuted locally Thursday by the Grant Park Symphony Orchestra; the featured soloist is Fritz Kaenzig, a top-notch tubaist who teaches at the University of Michigan. The program includes two other rarities: Poem for Orchestra (1944) by William Grant Still, the dean of African-American composers, and Rachmaninoff's Second Symphony. Written with polish and conviction, Still's work is a paean to impending Allied victory in World War II, but conveys muted jingoism. Often dismissed as an overlong stretch of maudlin romanticism, the Rachmaninoff is actually a logical, richly nuanced extension of Mahlerian sentiments. Isaiah Jackson conducts. Thursday, 7 PM, Petrillo Music Shell, Grant Park, Columbus and Jackson; 742-7638.