In 1976 the Chicago String Ensemble opened shop as the midwest's only professional all-string orchestra. Now, several ups and downs later, it celebrates a milestone anniversary--still the midwest's one and only string orchestra and one of a handful in the country. Aptly, the ensemble will open its new season reprising the very first program it performed. The program includes Samuel Barber's Adagio for Strings and Bach's Brandenburg Concerto no. 3. The best bet, however, is likely to be Benjamin Britten's song cycle Les Illuminations. Using texts from transcendental prose poems by the great French symbolist poet Rimbaud, the songs (originally written for Britten's friend tenor Peter Pears) are hauntingly beautiful--featuring opulent accompaniment from the strings. Rounding out the mostly modern program are Hindemith's Five Pieces and Two Elegiac Melodies by Grieg. Soprano Maria Lagios will solo. Tonight, 8 PM, St. Paul's United Church of Christ, 2335 N. Orchard. Saturday, 8 PM, Hammerschmidt Chapel, Elmhurst College, 190 Prospect, Elmhurst. 332-0567.