Great chamber music playing is supposed to be about a seamless fusion of separate musical parts, a harmonious democracy in which individual freedom is sacrificed for the good of the whole. But the thrill of the Alban Berg Quartet and Tabea Zimmermann's performances of Mozart's late string quintets, K 515 and 516, was the way their five separate personalities became part of the fabric of the music: the unpredictable phrasing of the leader, Günther Pichler, the sonorous pizzicato of cellist Valentin Erben, and the warmth of Zimmermann's viola playing. What emerged from this discourse among musical equals was not a Mozart of serene perfection, but a composer of volatile, explosive imagination.

The first movement of the C major quintet generated a symphonic power, developing from the smallest of musical gestures into a massive, complex structure. Zimmermann turned the slow movement into a virtual viola concerto, unfurling a seemingly endless musical line that began as a simple melody and blossomed into an ornate fantasy. The players made the menuetto third movement a succession of strange gestures and fragments, with weird chromatic lines and syncopations in the middle section.

But the G minor quintet contained even more extreme juxtapositions of musical style and substance. After the lyrical intensity of the first two movements, the adagio slow movement opened another emotional world. Playing with mutes, the five players created a sound world of delicacy and intimacy, as Pichler suspended a halting, song-like melody above the gossamer threads of the four other parts.

The slow introduction to the finale, unique in Mozart's output, wrenched things back from this mysterious world with music of tragic, minor-key passion, symbolised by the cello's violent pizzicatos. The moment when this music melted into the ebullient, major-key allegro was a startling contrast between two expressive worlds, brilliantly realised by all the players.