That coziness with musical profundity that has characterized the Cleveland Quartet's superb appearances in the Corcoran Gallery's little Armand Hammer Auditorium is threatened. Too many people have found out about it.

This week dozens had to be turned down when they phoned in for seats to last night's all-Beethoven concert -- all-Beethoven being the top of today's chamber music hit parade. Folding chairs came out. Subscriptions are mounting. And the Corcoran's Jane Alper, a lovely person without whom this all probably would never have occurred, is wrestling with whether success might spoil the special appeal of this series. at which About 200 listeners sit in a colonnaded little lyc'ee-like room that makes even the Juilliard Quartet's quarters at the Library of Congress seem gigantic.

And it is a sad duty to inform those who could not get in last night that they missed a wonderful concert.

The members of the Cleveland are about to fly off to Japan to do a complete cycle of Beethoven's 16 quartets -- the richest single achievement in the whole chamber music repertory. And last night they played three of the ones less often heard, even in this city where the Juilliard gives us lots of all-Beethoven programs every year.

The highest of last night's many high points came in the exalted slow movement of the E-flat major quartet, Op. 127. It is the first of that group of late quartets that changed the course of music for about a century. It is prayerful in mode, dominated by a melody of breathtaking eloquence. The rapt, hushed playing of that melody, especially by first violinist Donald Weilerstein, conveyed magnificently the ambivalance between lyric ecstacy and metaphysical mystery that this realm of art is all about.

There was also another big quartet, the F-minor, Op. 95 ("Serioso"), as well as the splendid first one, Op. 18, No. 1. Both were memorably done.