Now 16 years old, the Brentano String Quartet must be one of the most euphonious outfits on the planet. Balance and intonation reach an almost unearthly level of perfection, yet there's nothing inhuman about these American musicians. And their intimate embrace is marvellous: it's as though each player is inhabiting each others' skin. This was so at the Wigmore Hall, even with a change of cellist - though since Michael Kannen was their cellist until 1999 you could say he was only returning home.

The group have made a speciality of turning starry specimens of pre-classical vocal music into music for string quartet, and this time it was Monteverdi's turn, with four mostly dolorous madrigals (from the sixth book) coursing through love's torments in succulent dark colours, sweetly spiked by Mark Steinberg's first violin. Without the words we might expect to enjoy half the experience, but we still got the picture, thanks to Monteverdi's skill in knitting emotion into music and the Brentanos' plangent clarity.

Haydn's D quartet, Op 76 No 5, offered its own delights. The German word gemütlich was coined for these players' genial tone, though some might quibble that their relaxed deportment was taken a step too far. Extra fire in the finale, more pointed articulation in the minuet: that's what I wanted, anyway.

After the interval, Bach's incomplete final fugue from The Art of Fugue sat on their instruments as firmly as had the Monteverdi. Then came Bartók's Quartet No 6, and the concert's question mark. The players caught its tugging pain and melancholy - audible from the start in Misha Amory's tremulous viola. And venom erupted, as it should, in the third movement's burlesque.

Yet in other respects the performance appeared oddly veiled. The Hungarian snap might not come naturally to American musicians, but this 1939 quartet is Bartók's farewell to his homeland and to have its national rhythms soft-pedalled was a little eerie. But even Homer nods; the Brentanos are still a splendid quartet.

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