The Boxhead Ensemble has had such a symbiotic relationship with Braden King and Laura Moya’s bleak, elegiac 1997 documentary, Dutch Harbor: Where the Sea Breaks Its Back, that it wouldn’t be unreasonable to assume the group had no viable agenda of its own: of the ensemble’s first three records, one doubles as Dutch Harbor’s sound track, and the others were recorded live while the band accompanied screenings of the film. But the group’s fourth album, the brand-new Two Brothers (Atavistic), has nothing to do with Dutch Harbor, or with any other film. Instead of using images as source material, the band creates its deliberately paced, gorgeously textured improvisations by abstracting Civil War-era ballads, hymns, and folk songs: one tune borrows from “When Johnny Comes Marching Home,” for instance, and Stephen Foster’s “Hard Times” is embedded in the penultimate track, “Come Again No More.” From the beginning the Boxhead Ensemble’s fluid lineup has been composed mainly of musicians drafted from Chicago’s avant-garde and rock communities by its director, multi-instrumentalist Michael Krassner; this version, a sextet, features violinist Jessica Billey and cellist Fred Lonberg-Holm, whose melancholy bowing, ringing with overtones, gives Two Brothers both emotional immediacy and a surpassingly spacious feel. Billey and Lonberg-Holm stick pretty close to the original themes, but the album’s guests (nine in all) take the music to places Johnny Reb and Billy Yank could never have imagined. On the 18-minute title track, Jeff Tweedy’s electric guitar solo starts out sounding like he’s tearing off his strings, and by the end people who think he goes out on a limb in Wilco will probably figure he’s sawed off the bough behind him. At the other extreme is a delicate chamber piece, “The Half-Light,” with clarinetist Guillermo Gregorio and guitarist Jeff Parker: over a field of resonant strings and swelling percussion, Gregorio plays painterly variations on a simple figure, ornamented by Parker’s quietly dissonant chords. For this show the Boxhead Ensemble will be Lonberg-Holm, Krassner, drummer Ben Massarella and guitarist-organist Tim Rutili from Califone and Red Red Meat, and former Souled American guitarist Scott Tuma. They’ll again accompany a film, but for the first time it won’t be Dutch Harbor; instead they’ll play along to selections from King’s minimalist series “Films for Live Performance,” which he’s screened during concerts by Papa M, Brokeback, and Low. Saturday, September 8, 9 PM, Abbey Pub, 3420 W. Grace; 773-478-4408.