Zehetmair Quartet, Queen Elizabeth Hall, London

Founded in 1994 and vastly praised for its recordings on the ECM label, the Zehetmair Quartet remains, against all the received wisdom, a part-time outfit: rehearsing and touring just one new programme a year.

Founded in 1994 and vastly praised for its recordings on the ECM label, the Zehetmair Quartet remains, against all the received wisdom, a part-time outfit: rehearsing and touring just one new programme a year. Moreover, as the violin virtuoso and conductor Thomas Zehetmair leads his austerely black-clad colleagues onto the stage, one sees only a single chair and no music stands at all. Not only do all but the cellist stand throughout; they play their entire concerts from memory. But least conventional of all is the way they play.

This particular programme was all of "late" music, beginning with Bach's Chorale Prelude on "Vor deinen Thron tret ich hiermit" ("Before Thy Throne I Now Appear"), BWV 668a, attached to the unfinished score of The Art of Fugue. After negotiating its intricate network of canons by inversion with a slightly numbed sobriety, the Zehetmairs launched without pause for applause into the still more tremulous opening cross-pulsations Britten's String Quartet No 3, Op 94.

There is no question that the dying composer intended this last major work of his life as a testament. And the Zehetmairs' exceptional fragility of approach suited the unworldly quality of much of the opening movement with its exiguous duets - still more, the remote stasis of the stratospheric central slow movement. Nor was there any lack of contrasting gutsiness in their renderings of the implacably grinding patterns of the second movement or the burlesque savagery of the fourth. But when they proceeded to play the long valedictory passacaglia finale, with its resonances of Venetian bells, in a disembodied pianissimo, largely suppressing Britten's louder dynamics, one began to wonder at what point extreme sensitivity passed into mannerism.

The question became still more pressing in their performance of Schubert's amazing String Quartet in G major, D 877 - his last and largest, completed just two years before his death. Here, the stark contrasts, not only of major and minor tonality, but of loud and soft dynamics, upon which the entire 45-minute structure turns, are boldly flung down in the jagged opening bars. But once again, the interpretation we heard - a few rough outbursts apart - proved almost perversely biased towards the more fugitive end of the spectrum. Indeed, a lot of the playing seemed to be bowed sul tasto (over the fingerboard, producing a more muted sound).

The result was to deprive the outer movements of much of their driving obsession, while reducing to tentativeness the more songful lines of the inner movements. True, the approach occasionally threw a ghostly new light on some of the more visionary harmonic progressions, but for much of the time it was like listening to Schubert recomposed by Ligeti. This was one of the strangest quartet concerts I have ever heard.

Concert broadcast: 2.30pm, 9 May, BBC Radio 3