The Smithson String Quartet broke a promise last night in the Hall of Musical Instruments. It will be kept next season, no doubt splendidly, but it won't be the same.

All the quartet's programs, since the season began last October, said that the Smithson would conclude its season with Schubert's intense, deeply probing "Death and the Maiden" Quartet, one of the supreme masterpieces of the form. Instead, the program ended with Beethoven's genially eccentric Quartet in G, Op. 18, No. 2, no match for "Death and the Maiden."

Why the substitution? The Smithson Quartet is getting ready to record Beethoven's six Op. 18 quartets; it has become an internationally known ensemble and has recordings planned on three labels: London/Decca, EMI and the Smithsonian's own prestigious label. It is good to see the group getting such hard-earned recognition, but it is not good to see commercial considerations intruding on a carefully planned and balanced program. Rock groups or classical superstars like James Galway are expected to promote their recordings in concerts. But it is disturbing to see this commercialism contaminating a chastely classical group dedicated to performing old music on authentic instruments under the scholarly auspices of the Smithsonian.

The Beethoven was by far the best music and the best performance on the program. The Smithson Quartet is ready to record this music, but it should work a bit more on the two early Schubert quartets that opened the program before it tapes them in London this weekend. Last night's performance, while all right for a live concert, had little flaws of intonation and ensemble that will not work in a recording.

The Schubert works are bright, and inconsequential -- the work of a gifted teen-ager writing for performance by his family at home. It is no surprise that Beethoven in his late twenties, writing for publication, produced better quartets. If the concert had ended with "Death and the Maiden," it would have offered an interesting contrast between the young and the mature Schubert. But playing two immature quartets on this program was pointless, except that the ensemble is about to record them and needs the practice.