The Either/Orchestra is presently celebrating its 25th anniversary with the release of its first new studio CD in seven years, Mood Music for Time Travellers (Accurate). One person who never anticipated such longevity is the band’s founder and leader, Russ Gershon. “It was just going to be a Monday night rehearsal band,” he says from a car en route to Boston, the E/O’s base since its inception amidst the city’s vibrant club and college scene. “It was going to be fun-we’d get together and drink beer and play charts. I had no intention [for it] to become a working band. But I learned quickly that without a gig to practice for, people were going to lose focus.”

Countless gigs and 11 albums later, the 10-piece ensemble has shape-shifted continually. The E/O’s sound-originally modeled after the expansive, multitextural creations of Gil Evans, Sun Ra and Duke Ellington-has, over the years, absorbed swing, Latin elements and, perhaps most uniquely, Ethiopian music. Gershon plays tenor and soprano saxophones and writes most of the E/O’s material; he and trumpeter Tom Halter are the only remaining full-time members from the initial lineup (although early baritone saxophonist Charlie Kohlhase has returned for the new album). Both Gershon and Halter attribute the Either/Orchestra’s tenacity to its ability to redefine itself each time a new player enters the fold. “Everybody who’s come into the band has brought something to the band and feels appreciated,” says Halter. “So it’s always been a real positive experience. … I can’t think of a single gig that was a drag.”

“I always make sure everybody gets featured,” adds Gershon. “I think that works out well for the listeners because everybody’s got a different personality. It’s not about anonymity and interchangeability. It’s like a little opera where there are characters.”

Among those “characters” are several musicians who have since become jazz headliners in their own right, including drummer Matt Wilson, who played with the E/O from 1990-95; MacArthur-winning alto saxophonist Miguel Zenón, with the band from 1997-98; and John Medeski, who played keyboards on the E/O albums The Half-Life of Desire (1989) and The Calculus of Pleasure (1992), and has since risen to prominence with Medeski, Martin and Wood. “The band was really crossing genres in an amazing way, not stuck in any format, and, for me as an accompanist, playing behind all the different soloists was a real treat,” says Medeski. “Everyone actually listened to and interacted with the rhythm section, something that is missing in most ‘jazz’ players these days. Plus, they had very different styles, which made it possible to enter a different universe with every solo. E/O kept the independent spirit going, back when neo-classic, young-lion imitation jazz was all the rage in Boston, and everywhere. They were totally fearless.”

“There was a real sense of community behind what we were doing,” adds trombonist Josh Roseman, an original member of the band in the mid-’80s. “We were there to participate in each other’s development and to make good music. It was really kind of special. Russ has respect and a real understanding of the value of the musicians.”

From the outset Gershon aimed to incorporate a wide range of influences into the Either/Orchestra. And he did so with flair, balancing serious chops and courageous experimentation with a strong sense of entertainment and subtle humor. “Even though we would hit a lot of terrains and environments in one set, it always made sense,” says Wilson. “Russ is really great at programming a set of music and really great at allowing people to play. It’s not an easy thing to do with that large a group. And Either/Orchestra has given people opportunities to get some experience, to get out and play and learn.”

Through the years, the E/O has certainly undergone its share of transformations, most notably the incorporation of Ethiopian grooves. That productive obsession worked its way into the mix in ’90s, and was eventually documented with a three-song suite on the band’s 2000 album, More Beautiful Than Death, and on 2004’s Live in Addis, part of the acclaimed Ethiopiques compilation series.

The latest album, Mood Music for Time Travellers, leaves behind the Ethiopian influences for now. Having accumulated a slew of new compositions during the band’s recording hiatus, Gershon, as always, chose to accentuate the strengths of the current configuration. In this case, that meant a new look at the past, including the Latin influences that marked the band’s 2002 Afro-Cubism album. “This is a good cross-section of the original writing in the band over the last six or seven years, but a lot of it sounds sort of retro,” Gershon says. “There’s definitely a ’60s or ’70s vibe to it. Also, I had just gotten this mint-condition, old Hammond organ, and that’s got such an amazing ’60s sound. It suggests a certain thing in the music, so a lot of these tunes were written at the Hammond. But maybe that’s the ‘mood music’ thing, too-there’s a very buoyant quality. A lot of the past albums have had more obvious dark streaks. I’ve been through enough bad, weird moods, so maybe now that I’ve gotten to be 50 years old I want music that’s gonna put me in a good mood.”

He deserves it. After all, for 25 years the Either/Orchestra has given so much both to audiences and those who’ve passed through its ranks. Says Medeski, “I had mixed feelings about the band when I first heard them. But when I played with them, which is always the real barometer, it was so much fun and felt so natural that I fell in love. It was with E/O that I first saw people out there, in the U.S., who wanted to hear jazz or improvised music. When all of our mentors were going to Europe to make money, Russ booked some tours in the Midwest, and people came out and stayed until the end. That confirmed for me that there is no reason to sit around waiting for something to happen for you. You just get out there and do it.”

The Either/Orchestra will have a 25th Anniversary Concert in New York at Le Poisson Rouge on Feb. 11. All of the players quoted in this place will be there, as well as 20 other band members and alumni.