Often noted as the best local example of a new breed--the conductorless chamber ensemble--Concertante di Chicago is also remarkably imaginative in its programming. In this salute to two titans of American music, the group has picked a couple of relatively early obscurities from the composers' oeuvres. Aaron Copland's Quiet City is a modest charmer long overshadowed by the rambunctious ballet Billy the Kid. It shares the Copland half of the bill with Appalachian Spring, written in 1944 for Martha Graham and her dancers. Leonard Bernstein's Trouble in Tahiti (1952), receiving its debut professional performance here, is a sardonic, tuneful one-act tragicomedy about life in suburbia. His first opera, it was later incorporated into his last, A Quiet Place (1983), considered by many to be his most important classical work. Copland was a marked influence on Bernstein; both wrote in masterfully eclectic idioms and in many genres. This concert should prove that even early in their careers they knew how to grab an audience's attention. With mezzo Catherine Stolz and bass-baritone Paul Kreider. Sunday, 3 PM, concert hall, DePaul University, 800 W. Belden; 993-7887.