Now, 63 years and nine personnel changes since its formation, it remains a force in American chamber music - particularly with the recent addition of Nick Eanet, former Metropolitan Opera concertmaster, who was heard locally for the first time at the Philadelphia Chamber Music Society concert Tuesday at the Kimmel Center. But years of inevitable change have yielded a Juilliard Quartet requiring patience and time; some members have been with the group for 40 years, and it shows. The value of accumulated wisdom isn't so simple in a medium where flaws are so mercilessly exposed.

Anything louder than a mezzo-forte lost color and definition. Some incidental solos midway through Schubert's "Rosamunde" Quartet (D. 804) had such a loosely wrapped vibrato and evidence of fatigue that you feared for such still-to-come heavyweights as Bartok's String Quartet No. 2 and Beethoven's String Quartet in F (Op. 135). Long passages of less-than-purposeful tempos and questionable intonation had to be overlooked, though attentive listeners were rewarded with passages of astoundingly sure insight.

During the Schubert, any number of phrases were weighted with a long-considered sense of tension and release that made the music speak so firmly, persuading you that this is the way the piece should always be. Beethoven's Op. 135 shows the composer, after years of gradual retreat from the outside world, expressing his own brand of hard-won resolution. Most performers are heard probing the piece from the outside; the Juilliard reading exuded inside understanding, particularly in the pulsating slow movement.

Certainly, Eanet provided a solid anchor. So much about his playing reflects the best of the Met's longtime music director, James Levine - a tone with depth, luminosity but transparency that lets you go below the sound's surface. Then there's that effortlessly operatic sense of line.

That, plus the quartet's sense of blend - more pronounced than in the firebrand years of the past - meant that Bartok lacked the Juilliard's trademark sharp, well-defined contours that underscore what an artistic breakthrough the piece was for the composer. Other quartets now project that quality; the Juilliard view looks backward from the Elliott Carter generation and finds surprising consolidation in Bartok's musical elements.