Emerson String Quartet. (Lisa Mazzucco)

It’s always interesting to see what happens when a music ensemble makes a significant shift: a new music director, a new quartet member. Indeed, the latter has provided fodder for at least two feature films, including last year’s “A Late Quartet,” which happened to be released just as the Emerson String Quartet, one of America’s longest-established ensembles, was in its final season with David Finckel. The musicians had played together for 34 years. On Wednesday, the quartet came to the Terrace Theater, courtesy of the Fortas chamber series, for a debut of sorts; it was its first Kennedy Center performance with its new cellist, Paul Watkins. And the group showed that change is good: The Emerson has turned in reliably fine performances over the years, but this concert had a vitality I haven’t heard from the quartet in a long time.

This is no criticism of Finckel. Indeed, the Emerson Quartet has always been somewhat bottom-heavy: Finckel, slightly younger than the other three, was an animating, driving force, and it was hard to imagine how they would sound without his impetus, his solid sound, his alert presence, the latter expressed through little birdlike darts of eyes and head.

Watkins, it turns out, fills the role perfectly. He, too, is a little younger than his three colleagues, and he, too, acted as an animating force. More than anything, he sounded gorgeous. So the quartet is still bottom-heavy; on Wednesday, the other three players seemed to be adjusting to a new level of sonic opulence — not heretofore their stock in trade. The presence of a new player in a group that has played together for so long naturally leads to creative readjustment, to rehearing and rethinking of even familiar music. To judge from this concert, this process, for this group, is proving a fertile one.

The quartet had a wonderful showcase in its program, which balanced two intense and valedictory works in the first half with Beethoven’s “Razumovsky Quartet” (Op. 59, No. 3) in the second.

The performance opened with Mendelssohn’s dark, aching Quartet in F minor, the last work he wrote before he died, in which the composer’s wonted sunniness is turned to angst, his lyricism twisted into skittering and scrubbing of strings — a kind of anxiety at which the Emerson seems to me to have excelled in the past, here given an extra ache through the mellowness of the cello.

Following this was another final work: Benjamin Britten’s last quartet, the Third. Where Mendelssohn’s piece represents a new marshaling of energies, a creative step forward that was cut off by a young death, Britten’s is autumnal, its colors thinned and clear, drifting into compact episodes, culminating with a long reminiscence of his last opera, “Death in Venice,” its voices assigned to different instruments. The music is at once transparent and rich, and the Emerson did it marvelous credit, from the first movement — vivid and slightly creepy, pairs of voices slithering around each other — to the eerie dance in the fourth movement, with wiry pizzicati and plump oompahs from the cello punctuated by little squirmy questioning figures.

You could say it was a conventional program, with the newer piece flanked by two canonical ones, returning at the end to solid Beethoven. But it felt rather like a musical journey to a new place — or a meditation on the benefits of a new perspective. Violinist Philip Setzer often sounded more competent than lyrical in the first half, and Eugene Drucker, the other violinist, sometimes seemed a little under the pitch in the second half, while violist Lawrence Dutton and Watkins shared a warmth between them. But in the third movement of the Razumovsky, there’s a repeated passage in which the line is handed seamlessly from one instrument to the next, from Drucker down to Watkins, and at that moment the players sounded like a single instrument. It’s one I look forward to hearing again.