Labels in music are hopelessly inadequate, yet there’s a reason we keep turning to them. They are useful, for instance, in indicating what kind of concert it was that the two excellent young groups Victoire and the NOW Ensemble offered at the Atlas Performing Arts Center on Saturday night, courtesy of the Library of Congress. “Contemporary music” evokes visions of serialist knottiness; “alt-classical” better conjures up the mood of new classical music with an indie-rock sensibility, combined with amplification and a synthesizer or guitar, that prevailed on Saturday night.

The audience, I think, is still about equally divided between those for whom this kind of classical music rock band is still something of a curiosity and those for whom it’s become so well established that it’s silly to call it a new phenomenon at all. Yet the groups themselves are still slightly caught up in questions of identity and presentation. Even on a darkened stage with spotlights, amps and cables snaking between their music stands, they performed with the formality of classical musicians (the NOW Ensemble’s electric guitarist wore a sport jacket). And composer Missy Mazzoli, Victoire’s much-praised founder whose music was played by both groups, made a few comments to the audience but neglected to say anything at all about the music the band was playing, letting the works go by without title or identifier beyond mentioning that they were all on Victoire’s new CD (including the title track, “Cathedral City”).

The music itself was pretty terrific. The NOW Ensemble offered four tracks by four different composers from its new album “Awake,” released in April. Each had a distinct voice but a shared ethos, in part because of the distinctive instrumentation of flute, clarinet, electric guitar, piano and double bass, which leads to a lot of piercing or clarion top lines and a lot of layering of sound as the disparate timbres intersect.

Mazzoli’s “Magic With Everyday Objects” opened the NOW Ensemble’s set, a thoughtful piece with an easygoing piano line leading through wistful storms of sound, the winds keening, the amp from the electric guitar offering a buzzing haze of static. Closing it was “Change” by one of the NOW Ensemble’s composer members, Judd Greenstein. An antic piece with repeated, shifting patterns that snap together like building blocks, it was played alongside a film by Joshua Frankel called “Plan of the City,” starring the members of the ensemble, in which the buildings of New York shoot rocket-like to Mars. This proved one of the best matches of visuals to music I’ve seen: The film’s low-key jokiness was tailored to the loose conversational tone of Greenstein’s music, and neither overpowered the other. Judge for yourself: You can see the film on Frankel’s Web site and hear both the NOW Ensemble’s album and Victoire’s on the Web site of New Amsterdam Records.

Having already succumbed to Mazzoli’s “chamber-rock” sound world on record, I wanted Victoire’s performance to take me a little further into it than it did in person. Victoire has two keyboards, a clarinet, a violin and a double bass, plus electronic effects, recorded sound, toy instruments and the voices of its own members, all drawn in on occasion to add particular colors to a very particular sonic vision. There’s something ethereal about Mazzoli’s and Victoire’s sound, though it’s not in the least delicate: The group plays with all the toughness of a traditional girl band. Its signature sound can be found somewhere between the sustained straight tones of “The Diver,” punctuated by the scrubbing bowings of the violin; the kinetic click-track recorded accompaniment of “A Song for Arthur Russell”; and the high, aggressive string harmonics that close out “India Whiskey.”

In a nearly awful twist to the avant-garde trope of smashing pianos, a misstep before the show sent Olivia De Prato’s violin crashing to the floor, causing a gasp of horror from the audience. Fortunately Victoire played on in spite of, rather than because of, the destructive impulse.