Shape-shifting ensemble offers string sextet fare

Concertante isn’t so much a fixed ensemble as a pool of young, accomplished musicians who perform in combinations, from quintets to nonets. Yet for its Coleman Chamber Concert in Caltech’s Beckman Auditorium on Sunday afternoon, Concertante materialized as a string sextet -- and only a string sextet.

Groups organized solely for the sextet literature are pretty rare. Usually we get established string quartets with pairs of guests attached, ad hoc festival groups or celebrity soloists billed as so-and-so “and friends.”

Such gatherings of high-powered colleagues excited by unusual (for them) repertoire and each other can strike real fire. This one, though, based at the Whitaker Center for Science and the Arts in Harrisburg, Pa., burned on a steady medium flame.

Concertante on this given day was composed of Xiao-Dong Wang and Ittai Shapira (violins), Ara Gregorian and Rachel Shapiro (violas), and Alexis Pia Gerlach and Sophie Shao (cellos). It was pretty much a meeting of young, earnest, highly professional equals -- well-balanced dynamically, with Wang just a bit showier than the others. Thus, there was a solid, high floor of competence in this sextet, but the musicians couldn’t soar beyond a certain level.

This was apparent in their performance of Schoenberg’s “Verklarte Nacht” -- this wonderfully ultra-sensuous, ultra-Romantic masterwork that still pleasantly surprises audiences expecting a cerebral, dissonant bed of nails. The intimate original sextet version heard Sunday can leave you spellbound, as the old Hollywood String Quartet recording supervised by Schoenberg still does. Concertante’s performance was nicely slow, immaculately executed, just lush enough in texture, yet you wanted them to let go and squeeze more emotional juice from the notes and markings.

Likewise, the Brahms String Sextet in G, Opus 36, was unified, balanced, often lovely and rather restrained, although the dashing Gypsy-flavored Trio of the Scherzo couldn’t help but provoke an animated response.

However Concertante showed a glimpse of its programming savvy by coming up with a fascinating preface for Schoenberg in Martinu’s String Sextet. It’s mostly a vigorous, optimistic piece -- neoclassical with a Czech accent -- yet fitted with a deceptively gloomy introduction that was strikingly similar to the opening bars of “Verklarte Nacht.”