With the assistance of violist Lawrence Dutton, the Juilliard Quartet chose an exceptionally interesting program at the Library of Congress last night to show off the heightened responsiveness and depth of the recently reconditioned Stradivari instrument collection.

The full measure of the change came to the fore in a Quintet written in 1894 by the relatively unknown Charles Loeffler, an American composer and violinist of Alsatian birth. Scored for the unusual combination of three violins, viola and cello, the work allowed all five of the Stradivari instruments given to the Library by Gertrude Clarke Whittall to be heard together. (The Juilliard's violist, Samuel Rhodes, made an amiable and capable shift to violin for the quintet.) Filled with chordal passages given an antique flavor by modal harmonies, the music received loving and expansive treatment from the players. They reveled in the rich sounds, coming close to overindulgence, but always saved by their own sense of proportion and the simple, fresh style of the writing.

Dutton, who has been violist with the Emerson String Quartet, made his sensitive presence known immediately with an exquisite line in the opening C-minor Overture of Schubert, a remarkable achievement from that composer's 14th year. First violinist Robert Mann, violist Rhodes and cellist Joel Krosnick filled out the first half of the evening with a charged performance of Scho nberg's Trio, Op. 45, a dense musical translation of his near-fatal illness in 1946. The concert ended with a spirited reading of Hadyn's genial Quartet, Op. 64, No, 6.