Bach's six solo cello suites are fiendishly hard to play, and not often programmed because they demand rapt, microscopic concentration from the audience. Every choice the cellist makes in phrasing changes the relationship of one motif to another and transforms the performance. Cellist Pablo Casals describes his introduction to these works when he came upon the sheet music: "I was 13 at the time, but for the following 80 years the wonder of my discovery has continued to grow on me."

The Dumbarton Concert Series presented cellists David Teie and Andres Diaz Saturday night at Dumbarton Church in Georgetown in a remarkable format. Teie opened with Suite No. 1 in G, and Diaz followed with Suite No. 3 in C; after intermission Teie played Suite No. 5 in C Minor, and Diaz closed with Suite No. 4 in E-flat.

Although both cellists are superb technicians, the only way to satisfy Bach's musical demands without occasionally losing intonation, or missing a note or two along the way, is to play it safe. Teie and Diaz went for the music behind the notes, and both captured it.

Teie spoke of the cello's almost human voice and its "hard-wired connection to ourselves" in Bach's music. Teie's bowing sang with a warm, oaky lyricism; Diaz bent phrases to their limit, then sprang them back on themselves. Both took exciting, recklessly fast tempos in quick movements, where Bach-affirming spontaneity took precedence over note-perfect precision.

There is almost nothing one would want from a string quartet that the Miro can't do: Clarity, articulation, balance and an appealing tonal palette are all accounted for. They seem to have been to quartet finishing school, and are professionally polished right down to the superficial but not irrelevant details of dress (snazzy suits) and deportment.

An impressively talented young foursome that opened the 24th season of the Dumbarton Concert Series on Saturday night, the Miro String Quartet has won several prestigious competitions, including the Naumburg last year and the Banff in 1998.

Their tone is well matched, neither too much like an organ, with a consequent loss of clarity, nor too reedy, with overly dry results. It is warm but lean, with enough flexibility for each of the players to add a little more vibrato or dynamic force and emerge, in turn, from the collective texture.

In the rarely heard opening movement of Samuel Barber's Quartet for Strings, Op. 11, the music calls for some occasionally forceful and bright open consonances to emerge from the agitated background like blasts of brilliant light; the Miro played these chords with gorgeous tone, good intonation and an almost aggressive luminescence. The effect was powerful.

Barber's first string quartet is a two-movement work (with a small tail at the end) that includes a Molto Adagio later recast as the famous Adagio for Strings. It is music more often put to service as a public dirge than enjoyed merely as the slow movement of a string quartet. On Saturday it did duty as both. A longtime volunteer for the Dumbarton Series, Bryan Jack, was aboard the airliner that crashed into the Pentagon on Sept. 11; the concert was dedicated to him and the Barber placed first on the program to honor his memory. The Miro played it "straight," neither downplaying the unsettled textures of the first movement nor inflating the lyricism of the second; the quartet played it as Barber wrote it, rather than warp the sentiments to fit the occasion.

Beethoven's Quartet No. 1 in F, Op. 18, No. 1, and Grieg's Quartet in G Minor, Op. 27, demonstrated a command of two very different regions of the repertoire. The Beethoven was played mostly on the balls of the feet, light and agile, relaxed and spare. The second movement, inspired by the story of "Romeo and Juliet," begins with an almost incorporeal sound; Daniel Ching's violin coaxed the melody slowly out of the gloom of a very refined pianissimo, while the rest of the ensemble created an even and hushed background. The effect was rather like the faint light of the solitary candles placed in the windows of the Dumbarton Methodist Church, flickering into consciousness with a bit of stealth.

When Beethoven revised his early quartet two years after first penning it, he removed some of the thematic obsessiveness of the first movement, but introduced a striking passage of two scale figures, played by all four instruments in unison and very loudly. It is a quintessential Beethoven gesture -- disruptive and dramatic -- that foils most interpreters. The Miro almost has it, but not quite. This detail, plus some rapid figuration for cello and viola in the last movement, and several larger, whimsical chunks of the Grieg, still sound undigested.

The Miro seems to share, collectively, a tiny failing that one sees often in very talented solo players. Being exceptionally good at the many things the world asks you to be good at can be a kind of straitjacket, diverting energy to the obligatory virtues and away from the small vices that add up to individuality. In the lighter movements of the Grieg (the second movement is really just a glorified bit of salon silliness), the players were less sure of themselves than in the Beethoven. And an encore, an arrangement of Jerome Kern's "Smoke Gets in Your Eyes" played a bit too cautiously only confirmed the impression. Everything has been mastered except what can't be: spontaneity and playfulness. It's a small thing and of no consequence if they're merely suppressing the will to fun while they attend to serious interpretation; but deny the instinct too long and it can wither.