Kronos Quartet, Barbican, London

Performing new music these days is such a breeze. Anything goes.

Performing new music these days is such a breeze. Anything goes. Audiences know that all they have to do is sit quietly and cheer wildly at the end of each piece. Gone are the days of boos, walk-outs and protests. Audiences have gone soft. But does it help the performers?

For some time now, Kronos has been dishing out the musical equivalent of Prozac. And worryingly, it begins to feel as if a composer wanting to be played by the San Francisco "Fab Four" must contribute to stylistic tics that seem firmly to have taken up residence. Above all, anything mournful will instantly propel a piece to the top of the "possible" heap.

In its 30-odd years, Kronos has commissioned more than 450 works and arrangements of works for string quartet. Of course, the miss-to-hit ratio is high: masterpieces don't grow on trees. But in its quality-control checks, has Kronos gone soft too?

The quartet's packed-out Barbican concert began with Michael Gordon's Potassium (a UK premiere). The most prevalent compositional tics of the evening quickly emerged: slithering slides, patterning, repetition and a sustained pedal in the cello. Lighting - from blue to white and back - delineated Gordon's sections, and (in retrospect) the work seemed well put together. The Azerbaijani Franghiz Ali-Zadeh's Oasis was also a well-organised piece; her delicate tracery of pizzicato and electronic water-dripping leavened by some lusty folk rhythms and whispered syllables, with quite a traditional harmonic framework, did not outstay its welcome - although heavy snoring to my left created formidable competition.

It was Stringsongs by Meredith Monk - receiving its world premiere as a co-commission between the Barbican and the Carnegie Hall Corporation - that shone out as a work of strength and character, despite her bleached, non-dynamically-contrasted, non-vibrato patterning, compositional tics of her own. Now in her sixties, Monk has earned her place in the sun.

Not so the Dutch composer Willem Jeths. Whereas the three previous pieces were short and structurally succinct, Trembling from Within (a UK premiere) massively outstayed its welcome, as uneasy coughing from the audience confirmed. Here was a piece of no coherence; a soup of styles - the opposite of the focused Monk - with de rigueur slithering slides and even an attempt, in a particularly insistent rhythm, to take on Beethoven. Kronos likes dark, mournful stuff, but Op 59 No 1 slow movement, rather than this.

Clint Mansell's Suite (arranged by David Lang), from his score for Darren Aronofsky's film Requiem for a Dream, was, again, slow and slurpy, static and mournful, throbbing (and occasionally pounding) in sub-Nyman clothes.

The American Alexandra du Bois is the first recipient of the Kronos: Under 30 Project. Her String Quartet: Oculus pro Oculo Totum Orbem Terrae Caecat ("an eye for an eye makes the whole world blind") could only be funereal, written as the Iraqi invasion was being contemplated. We were back to slow slithering.