Had it not been for Michael Haydn falling sick in 1783, the world might have been deprived of two small Mozart masterpieces, and a capacity audience at the Kennedy Center Terrace Theater Tuesday night would have missed out on one superb performance by members of the Guarneri String Quartet.

Of course, one wouldn't wish serious illness on anyone, but if history had provided a few more bed-ridden composers of mediocre ability -- and a few more composers of Mozart's talents standing on the sidelines -- we might today be enjoying more masterpieces of the caliber displayed in this performance of Mozart's Duo for Violin and Viola, K. 424 in B-flat.

John Dalley, violin, and Michael Tree, viola, gave an exemplary reading of such clarity and intensity that one couldn't help but hear the work's symphonic proportions. Even in the forthright Andante Cantabile that speaks so well for the viola's accompanying range, Dalley and Tree drew such a richness from the simple veil of notes that it was possible to hear an entire orchestra and not merely two instruments.

Erich Korngold's Suite for Two Violins, Cello and Piano (Left Hand), Op. 23, conveyed similar intensity if not proportion. Joined by pianist Gary Graffman (whose stunning control bound this performance together) and a third Guarneri member, cellist David Soyer, the ensemble crafted a high-voltage performance of stunning contrasts -- from the coruscating clarion call of the opening Prelude to the urbanity of the suite's waltz.

It is easy to see, on hearing Dalley, Tree and Soyer play Beethoven's Trio in G, Op. 9, No. 1, that the Guarneri Quartet sets the standard by which other ensembles might measure themselves.

The Scherzo was as light as the spume on a wave with just a hint of muscle in the Trio. And few quartets performing today are capable of playing the Adagio with such sculptured beauty, the Allegro's second subject with such breathy echo, or the closing Presto with such boundless joy and steely brilliance.