Something of a revelation, this disc, this programme – and, for the creator of Appassionato, Mathieu Herzog (late of the Ébène Quartet), something plainly close to his heart. Appassionato are a chameleonic chamber ensemble changing colour, complexion and size with the requirements of the repertoire. But as a string orchestra of some amplitude they really shine here, not just because they play like a string quartet – with soloistic precision, agility and passion – but because they actually comprise soloists from a number of well-known string quartets only too ready to answer Herzog’s call.

All three performances are exceptional. Herzog himself describes Strauss’s Metamorphosen as ‘a sort of farewell to the German culture he knew’, a valediction but also a realisation of the changing musical and harmonic landscape taking us to the very edge of tonality. The beauty of this piece is in its complexity, the way in which it draws the ear into ever more intricate layering of string lines, their intensity in complete accord with an emotional import second to none in all of Strauss. This is the piece he had to write and this performance – which seems to place us right inside the sound (the engineering has thrilling immediacy) – takes us deeper and deeper into his musical and emotional state of mind.

All three pieces are in some sense dark nights of the soul. Respighi’s Il tramonto (originally for voice and string quartet plus double bass or string orchestra) was new to me and the poem by Shelley which it voices – ‘The Sunset’ – is close in spirit to the transfiguring love/death meditation of Richard Dehmel’s ‘Verklärte Nacht’, which inspired Schoenberg. Herzog’s arrangement (or, one should say, ‘expansion’) deploys five actual quartets as one writ larger and the effect is at once seductive, spectral and elusive. Mezzo-soprano Adèle Charvet is most sensitive to the poignancy and twilit imagery of the text, and as we move closer to its transfiguration she and her string counterparts become more and more seraphic. Quite a discovery.

And so to the Schoenberg, which might be the best performance of all. I can’t recall a more hushed, breathless, furtive opening – and as the piece grows more febrile and impassioned, tranquillity rubbing shoulders with high anxiety, the precision (technically stunning) and high-voltage emotionalism of the performance become increasingly gripping. Real edge-of-seat stuff. There are startling pizzicatos and seismic basses but for me it is the quietudes, hushed and inward, that really resonate. I can hear now those traceries of string texture at the heart of the piece. Transfigured indeed.

All of a programme, then, brilliantly executed. What will this chameleon of an ensemble bring us next? Their name was well chosen.