Last night's installment of the Mostly Mozart Festival showed how much variety can be crammed into three hours of music-making with a string quartet and two guest artists. For early birds attending the 7:30 preconcert recital, the evening began with the slight but attractive Three Ballads for guitar from "El decameron negro" by Cuban composer-guitarist Leo Brower. At 10:30 the concert closed with the brilliant final presto of Beethoven's heaven-storming String Quartet No. 15 in A minor, Op. 132. The two pieces hardly seem to occupy the same universe, but despite its title Mostly Mozart has room for more.

Between these extremes, the program played by the Emerson String quartet with pianist Rudolf Firkusny and guitarist Sharon Isbin was entirely 18th century and heavily Haydn. Mozart was represented by his Piano Quartet in G minor, K. 478, a work that could dominate almost any program. It is music of great intensity, particularly in its first movement, and it balances drama, wit and pure lyricism in a style familiar from Mozart's piano concertos, which it also resembles structurally. The performance was almost ideal, with a most satisfying sense of give-and-take between the piano and the strings, perfect balance and a fine, shared sensitivity to the music's rapidly shifting moods.

There was less challenge but undiluted delight in the two Haydn works that filled out the program. The playing was fleet and brilliant in the String Quartet in G, Op. 54, No. 1. One wanted sometimes to protest the Emerson's high-speed conception of what Haydn meant by "allegro," but the music was perfectly played at its breakneck pace. Tempos were only slightly more moderate in the Quartet in D, Op. 2, No. 2, where Isbin's guitar took the part normally given to the first violin. Here (as earlier in the Brower), one might regret the discreet use of amplification for the superbly played guitar, but it sounded completely natural and kept the instruments in an ideal balance.