While some other musical organizations in town are retrenching, the Chicago String Ensemble, under the direction of Alan Heatherington, keeps coming up with bountiful samplers of string music at its most edifying and refreshing. This all-American fare includes not only old favorites but also brand-new pieces: Leon Stein's Oboe Concerto and Oscar Haugland's Interlude, both to be performed for the first time, are by local boys; another premiere is the Prologue and Variations by Ellen Zwilich, an excellent if academically inclined craftsman often touted in the media as our foremost woman composer. Of the oldies, Norman Dello Joio's Meditations on Ecclesiastes, a Pulitzer Prize winner in 1957, is a bit dated, in my opinion, especially in the company of Henry Cowell's (inimitably unorthodox) Hymn and Fuguing Tune no. 10 and Samuel Barber's Dover Beach, whose wistful romanticism still epitomizes the (underrated) lyrical strain of 20th-century American music. Heatherington conducts a hard-working ensemble that can be both superb and erratic in the same evening. Featured soloists are: Michael Henoch, a Chicago Symphony oboist, and Robert Knight, a baritone with a solid reputation in the midwest. Tonight, 8 PM, St. Pauls United Church of Christ, 2335 N. Orchard; 332-0567.