PROMS / Soloists of the Moscow Conservatoire / Yuri Bashmet - Royal Albert Hall / Radio 3

It seems perverse to take a masterpiece for string quartet and rearrange it for the entirely un-intimate medium of orchestral strings. Yet the challenge fascinates many composers. When Mahler made his version of Schubert's Death and the Maiden Quartet - which opened Wednesday's Prom - he even achieved extra subtleties of sound, subdividing the violins and pitting solo against combined forces. You expect him, as a fastidious orchestrator, to have kept things lucid. The intriguing feature is that you also hear Mahler the analyst thinking aloud and using the double-bass not just for weight of tone but to underline musical turning-points.

Still, like most similar ventures, it ends up with the proportions sounding wrong. Crescendos happen too quickly; you feel the music needs to be extended in time, to match the expanded sonorities. It is no accident that quartets usually have shorter movements than the same composers' symphonies - or serenades, such as Tchaikovsky's, whose more relaxed form was a relief to turn to later in the concert.

Both pieces had performances of astonishing precision and refinement, with a pianissimo so delicate it was theatrical (the trick would be more devastating if used less often). Bashmet's new orchestra, with only one woman, can sometimes sound as severe as it looks, but it held its audience transfixed. He played solo viola in Schnittke's subdued but highly charged Monologue, and a delightful Vivaldi arrangement of his own as encore.