The 90th birthday of Pierre Boulez in 2015, his last before his death the following year, was marked by DG’s release of a 13 CD compilation, as well as by the Diotima Quartet’s version of Livre pour quatuor (Megadisc, 2/16) to compare with the Quatuor Parisii’s recording in the DG box. However, not even the Diotima’s 2016 recording was complete. Boulez had continued to work on a substantial fourth movement, and after his death Philippe Manoury undertook to devise a performable version, which is now recorded for the first time.

Manoury’s contribution fits well within the uncompromising ambitions and elaborate constructive principles of Livre pour quatuor as a whole. But it might also reinforce those doubts about the original piece that have been well rehearsed down the decades. Boulez’s doggedness in returning to scores to produce new versions yielded many rewarding results, but with Livre pour quatuor the doubts have continued. Even with the inclusion of Manoury’s characterful contribution, now recorded with needle-sharp fidelity to the music’s every sonic nuance, further clarity about Boulez’s own motivations, beginning the work in 1948 and failing to complete it in 2016, remains fascinatingly elusive, perhaps because critical discussion of it usually focuses on the title’s signalling of poetic connections with Mallarmé, something that would become increasingly important to Boulez in the years ahead. But in 1948, at the age of 24, he seemed no less intent on exploiting the fundamental tensions between the Gallic predispositions of one of his teachers, Olivier Messiaen, and the more Germanic orientations of another, René Leibowitz.

If these tensions are to be encapsulated in one work valued by both mentors, Alban Berg’s six-movement Lyric Suite for string quartet is an obvious choice. But Boulez’s response, replacing Berg’s high-modernist scheme with something much more avant-garde and multivalent, was a hugely ambitious project, and as much an intransigent dissection of high-modernist models as an act of homage. In Livre pour quatuor a six-movement plan underpins a sequence of nine sections, subdivisible in various ways, but with an emphasis on polarities, both reflective and aggressive. Music evoking the lyric or poetic, fleetingly romantic, is set against sounds that tend towards noise – percussive, questioning, even ridiculing rather than endorsing conventional string-writing.

There is nothing in such a scheme to show exactly why the fourth of the six movements, the seventh of the nine sections, should have proved so difficult for Boulez himself to compose. But there is something exceptionally intense about the music he did complete, suggesting another dramatic theme that aligned the Gallic with the Germanic – Pelléas and Mélisande. Even if it would be absurd to map the characters and events of Maeterlinck’s drama, Debussy’s opera and Schoenberg’s tone poem on to each other – perhaps adding the much narrower autobiographical drama of Berg’s Lyric Suite to the mix – the poignancy of the photograph in the Megadisc booklet of the nonagenarian Boulez working with the Diotima Quartet on music that seemed peripheral to his career-long achievement, yet crucial and necessary in ways which all those non-peripheral masterworks were not, is the ultimate endorsement of Boulez’s special relevance to a classical musical world now at risk of fatally slighting its essential experimental core.