The LaSalle Quartet, which played splendidly last night at the Terrace Theater, is one of the country's best and oldest chamber music groups. But for its 33 years it has performed here less than the other noted American quartets -- perhaps because it resides in Cincinnati, a little out of the East Coast chamber music orbit.

Last night it did not play quartets. On the program were two major string sextets, for which the LaSalle was joined by colleagues from the Cincinnati Conservatory, violist Donald McInnes and cellist Jonathan Pegis.

Two strengths for which the LaSalle is best known were displayed. One was its special aptitude for the music of the atonal Viennese school and its immediate antecedents. The pie ce de re'sistance was that wrenching last gasp of late Romanticism, "Verklaerte Nacht" (or "Transfigured Night"), by Arnold Scho nberg, the man who was about to put the knife to tonality.

The Scho nberg is considerably better-known in the version for string orchestra the composer arranged later -- and in that form is the music in Tudor's ballet "Pillar of Fire." But the greater clarity of the work in just six parts is alluring. The hallucinatory pizzicati in the violas and the fervor of the long-delayed resolution into a major key on a single cello sings out less fuzzily than from a full section of cellos.

Another strength of the LaSalle was its mellow blend, much more in the European model than the brighter sound we have come to know from the Juilliard, for instance. There is considerable urgency to the LaSalle's playing, without its ever seeming athletic. The quartet's ardent opening of the variations movement of the Brahms First Sextet, the other work on the program, was fully inflected without seeming jagged at the edges.

There were some drawbacks: occasional pitch problems, particularly in the violins during the Brahms; and first violinist Walter Levin seemed to be having a bad night, with thinner tone than the others. But in the lower instruments, where a sextet takes on a tonal dimension beyond a quartet, the LaSalle was distinguished.