They're back--and fittingly, we get to hear them on April Fools' Day. With their infectious musical humor and inveterate tricksterism, the Either/Orchestra both celebrate and subtly undermine the big-band tradition. They also remind us of how one artist's imaginative vision can drive an organization of his peers to giddily synergistic heights. Led by saxist/composer Russ Gershon, this ten-piece band from Boston retains an inventive, iconoclastic spark that manages to link such supposedly disparate spirits as Spike Jones and Charles Mingus. Because of its size, it's tempting to link the Either/Orchestra with the nonets led by Miles Davis and Shorty Rogers in the 50s and Lee Konitz in the 70s, or else to peg it as a combo with a glandular problem. But despite its size, the Either/Orchestra has clearly established itself as a big band--for that matter, one of the most adventurous big bands of the last 30 years. (If anything, Gershon has gone further back than the 50s for inspiration; it helps to recall that the earliest jazz orchestras contained as few as nine players, and that Duke Ellington's first masterworks featured "big" bands of only ten or eleven.) Just as the Either/Orchestra has compressed the 17-piece size and sound of modern jazz orchestras, Gershon's direction compresses jazz history. The Brunt (Accurate), their most recent and wholly impressive album, offers a case in point: it includes a variety of original compositions by Gershon and others, some lesser-known tunes by famous jazz composers (Ellington and Mal Waldron), and an absolute wild card--Bob Dylan's "Lay, Lady, Lay." Such juxtapositions indicate the band's postmodernist roots; to gauge their time-honored swing and splendid musicianship, you have to hear them in person, infusing Gershon's dense arrangements with power and light. Friday, 9 PM, and Saturday, 8 PM, Green Mill, 4802 N. Broadway; 878-5552.