A single performance does not a verdict make. But if Tuesday night's concert of Beethoven, Bartók, and Brahms at the Perelman Theater is a fair representation, the Hagen Quartet may in fact be the most gorgeously unified string quartet currently active.

The group's appearance here is rare. This was only the fifth time the Philadelphia Chamber Music Society has been able to present the Salzburg quartet, which travels to the United States spottily. They play on instruments once used by the now-retired Tokyo String Quartet, a set by Stradivarius known as the "Paganini" quartet, and the homogeneity is striking.

All of their work was on a very high level Tuesday night, but more than a handful of times they hit on a stretch that soared -- a fugal section that gathered steam, and a series of phrases where players built ideas off each other.

Passing material around in Beethoven's String Quartet in G Major, Op. 18, No. 2, they floated the illusion of being a single instrument with a large register. The elegance of this group could have been summed up entirely in the second beat of the piece. Beethoven asks the first violin to play an unusually elaborate ornament, a flash of eight notes that in other hands might have been a blur. Lukas Hagen, however, played it with such quicksilver ease that every note was discernible, yet all was strung together with shapely purpose.

Bartók's String Quartet No. 3 is muscular music, prone to bold strokes, but it was in the fine details of dynamics and articulation that the Hagen made its magic. The piece is full of unusual colors -- popping, buzzing, and slides -- but the players, most notably cellist Clemens Hagen, integrated the techniques into a burnished sound.

To hear these musicians play Brahms' String Quartet in A Minor, Op. 51, No. 2 was a fascinating experience to anyone who had learned the piece from the Tokyo's recording. The Hagen brought it a completely different character. It was all about connectivity and refinement. There were hot spots -- the eruptions amid peace in the second movement, for instance. I, for one, had always accepted the Tokyo's interpretation as beautiful, one of those recordings I'd replayed perhaps 30 times. But when you heard the way the Hagen Quartet made the first movement's phrases melt into each other, it had beauty for sure -- the sensation of rolling velvet -- but also the virtue of finally sounding right.