The strengths and weaknesses of the National Symphony Orchestra's Beethoven series could not have been better displayed than they were Saturday night at the Kennedy Center, when two 19-year-olds--violinist Hilary Hahn and pianist Jonathan Biss--took center stage for two glorious concertos.

So far, this season's festival suffers from the same condition that ailed past editions. The promise of a sold-out Concert Hall for a performance of undeniably superlative music--almost all of it standard repertoire that doesn't eat up a lot of rehearsal time--must hold irresistible appeal when programming decisions are made. It seems like a bold and boisterous way to kick off the season.

But this is a marketing decision, not an artistic one. The problems arise when the orchestra indeed does sound under-rehearsed, when the musicians themselves sound tired and passionless, and when fine young soloists receive only adequate orchestral accompaniment. Any chance for elevated dialogue between the heroic individual and the group--a basic premise of the concerto genre--goes out the window.

This was especially frustrating during Hahn's performance of Beethoven's Violin Concerto, which closed the program. Despite her age, Hahn is already a widely admired violinist, respected as a thoughtful and searching artist, and one never hears any sound come out of her violin that isn't sincere. She's technically fabulous, too, with a special rhythmic accuracy. But the NSO, under Music Director Leonard Slatkin, struggled to accompany her. As a result, the musicians would often not be together at the end of a long run.

Still, Hahn's playing was everything you'd hope to hear from a violinist of any age, and her exquisite, silky tone and strong personality drew the listener in. She made the first-movement cadenza (by Fritz Kreisler) sound like a weighty, stand-alone composition; at the moment when the two main themes overlap, near the cadenza's end, she mixed them with great deliberation and purpose, and I don't think anyone in the auditorium took a breath until she was finished. After the concerto, Hahn returned for an encore, the Allegro from Bach's Sonata for Solo Violin No. 2; her performance was a bit gymnastic in approach but sturdy and finely nuanced.

There was almost too much of a good thing on the program, as Biss played Beethoven's Piano Concerto No. 3 in an introspective and sensitive interpretation, never lapsing into self-indulgence. It was very quiet playing, like someone speaking softly and directly. In loud passages the contrasts were startling and held authority. The orchestra's rapport with the pianist was better than with Hahn, but it played with less musical exploration. I hope to hear Biss again soon.

The lovely but unremarkable opening work, the Romance Cantabile in E Minor, brought the orchestra's principal flutist, Toshiko Kohno, and principal bassoonist, Daniel Matsukawa, to the stage front. Slatkin played the piano part and bobbed his head to cue entrances when his hands were occupied.

The Beethoven series continues Friday with the Piano Concerto No. 4 and the composer's own transcription of his Violin Concerto, renamed the Piano Concerto No. 6. Garrick Ohlsson will be the pianist for both.