The second week of the Seattle Chamber Music Society's month-long Summer Festival concluded with a programme that – as the two earlier concerts that week had similarly done – expanded perceptions of the notion of chamber music itself by including works that cross over the instrumental divide and call for voice.

Overall, this is further evidence of the vision gradually being unfolded by James Ehnes in his capacity as the Society's artistic director: a vision that encourages fresh insight by juxtaposing the well-trodden repertoire with what, for many listeners, will be new discoveries. Thus, following a triptych of Beethoven quartets spread across the opening week (which he performed with his own ensemble, the Ehnes Quartet), a later programme offered a Respighi rarity: Il tramonto, the Italian’s lushly Romantic cantata setting of Shelley’s poem “The Sunset,” which is scored for string quartet and mezzo-soprano (sung with subtly shaded emotional contours by Sasha Cooke).

Friday’s concert opened each half with one of Britten’s Canticles (I and III), bringing back the young tenor Nicholas Phan after his thoroughly engaging performance on Wednesday as the ill-starred farmboy-seduced-by-gypsy in Janáček’s The Diary of One Who Disappeared. Phan’s manner of singing is holistic, encompassing not only a refined musicality but expert diction and compelling dramatic presence. It proved remarkably well-suited to the particular challenges of posed by Britten, whose music Phan has made a specialty.

Much as the Janáček achieved the impact of a miniature opera, Britten’s Canticle III (setting Dame Edith Sitwell’s “Still Falls the Rain: The Raids, 1940. Night and Dawn”) suggested a condensed modernist Passion. Phan used his ability to modulate from stentorian forte into a caressing sotto voce to movingly expressive purpose, both here and in the opening piano-vocal Canticle I (“My Beloved Is Mine”). The latter, with its ecstatic strands of incantation, highlighted the sweetness of Phan’s top range.

At the keyboard for both Canticles, Joyce Yang played with an evocative attention to detail — her role expanded in the interstitial instrumental passages that shape Canticle III, which also calls for solo horn. Jeffrey Fair’s eloquent contribution here followed on an admirable performance of the familiar Horn Trio by Brahms in the program’s first half. The Seattle Symphony’s principal horn player (another Ehnes stamp has been to integrate SSO players with the Chamber Society’s roster of visiting soloists), Fair was joined by pianist Jeremy Denk and violinist Yura Lee for the Trio.

Despite a few distractions from acoustic imbalance, theirs was a vivid account that teased out the enigmatic fluctuation between elegy and exhilaration in Brahms’s score. In the second movement, for example, the dark-hued trio section cast a mesmerising shadow over the rambunctious rhythmic accents of the Scherzo. Denk disclosed the Adagio’s opening theme with brooding introspection, and Lee’s expressively charged phrasing blended with Fair’s mellow horn to make a warm, rich sonic compound, touching but saccharine-free.

The most astonishing revelations were left for the final selection: an arrangement of Richard Strauss’s Metamorphosen for string septet. If this enhanced the impression of a largely somber chamber concert for a lovely summer’s eve, it also occasioned a performance of stirring, sustained beauty. Ehnes, who joined the ad hoc ensemble of pairs of violins, violas, and cellos with double bass, pointed out in his programme note that a short score of the late Strauss work, discovered in Switzerland in 1990, had been written out for septet (versus the 23 solo strings of the familiar version commissioned by Paul Sacher). Ehnes and colleagues performed a reconstruction of the reduced scoring Strauss may have originally intended before the commission; it was realised in 1994 by Rudolf Leopold, a Czech composer.

The differences were striking: not only, as you would expect, in the matter of flexibility and transparency (revealing new colours and even relationships amidst Strauss's intricate polyphony and barrage of quotations), but in the significance of individual voices that pull tauter the nearly nonstop weave of mournful, throbbing lyricism in Metamorphosen. At times Strauss almost seemed to be anticipating a kind of "texture music" of shifting edges and spectral tints.

Strauss famously alludes to the funeral march of Beethoven's Eroica, permeating the score's ebb and flow with tantalising fragments of his predecessor's theme, like a half-suppressed panic attack that continually threatens to break out again. When the quotation finally emerged in full low in the strings – with newfound clarity in this septet version – the effect was shattering.