"In the next 90 seconds," announces Wayne Horvitz, "I'm going to change the arrangement of this tune, so talk amongst yourselves." Not an easy feat, considering that Horvitz, along with Robin Holcomb and Tom Varner, has spent the evening piloting a big band through tunes that borrow as much from Duke Ellington and Igor Stravinsky as from John Coltrane and John Zorn.

Horvitz ducks between music stands, conferring with the trumpet section (where I spot Thomas Marriott and Reptet's Samantha Boshnack) and making sure that the trombones and the imposing front line of saxophones (which includes Mark Taylor and James DeJoie) know what to play next. Sheet music gets shuffled and no one seems to mind; the musicians are here to explore new territory, not rehash familiar numbers.

I'm happy to wait; the exhilaration of finally hearing WACO—short for Washington Composers Orchestra—at their monthly Sunday gig is unlikely to wear off anytime soon. WACO updates the New York Composers Orchestra, which swaggered through the Big Apple in the late 1980s with adventurous charts by Horvitz, Holcomb, Marty Erlich, Anthony Braxton, and others.

The soloists are superb; trumpeter Al Keith served up a raunchy, wah-wah-ing plunger solo on "Don't Stop Now (P.W.)" after which the band somehow stomped toward a whacked-out polyphony that ignited a saxophone battle between Eric Barber and Greg Sinibaldi. Alternating between a sympathetic duet and an old-school cutting contest, the pair stabbed, slashed, and parried with champed notes and whizzing, quicksilver scales. Horvitz's subsequent piano solo yoked greasy gospel licks to clusters jabbed up and down the keyboard. And that was just the first tune of the night.

If you're lucky, WACO will reprise Holcomb's "Nightbirds: Open 24 Hours," a wistful tone poem enfolding '70s-cop-show jazz, Gregorian chorales, and gritty, cacophonous eruptions from the entire band. I was also taken with Varner's "Old Man Trouble," a canny splintering of a familiar Ellington melody. Varner aptly describes his tune as "what if Messiaen had heard 'Cotton Tail' and flipped and tried to superimpose his clusters over 'I Got Rhythm' changes, and ended it with a dying Mahler feeling."

WACO (Sun May 31, ToST, 513 N 36th St, 8 pm, $5) is just one of several improvising big bands that meet early in the week. Two bands hold long-standing residencies at Tula's: the Fairly Honest Jazz Band (which trades spots with the Jay Thomas Big Band) and the Jim Cutler Jazz Orchestra (Sun May 31, Tula's, 2214 Second Ave, 3 pm and 8 pm respectively, $5). Up north, vocalist Courtney Cutchins sits in with the marvelously hued Jim Knapp Orchestra (Mon Jun 1, Seattle Drum School, 12510 15th Ave NE, 8 pm, $5/$10). recommended