The Kennedy Center Concert Hall audience was divided, on Saturday night, between those who were there because of the Chamber Music Society of Lincoln Center's performance of Lucas Foss' "Echoi" and those who were there in spite of it.

It was a marvelous occasion in every respect. At the end, as the percussion finally and irrevocably stamped the piano into submission, the pro-Foss contingent cheered and the anti forces booed lustily. The quartet of performers, which included pianist-composer Foss himself, waved triumphant fists in the air, like football players after a touchdown, and a good time was had by all.

The 25-minute, four-movement piece for clarinet, cello, piano and percussion is a true child of the '60s, the age of the "happening" (a word Foss uses to describe it). It is an event in which order and disorder contend, in which space is a leading character and in which the performers assume dramatic roles outside the music itself. It confronts the question "what is music?" squarely and then answers it paradoxically in terms the composer calls "not-yet-music."

The piece forces the listener to accept it on its own terms or not at all, which is fair enough. Those willing to go along with Foss are treated to a virtuoso display of creative imagination, wit and musical craftmanship.

The program was almost too rich for comfortable digestion: splendid performances of a C. P. E. Bach trio sonata and the second of the Mozart piano quartets before the Foss, and the Brahms String Quintet, which, coming after the Foss, was hard to concentrate on.