When an artist states his CD is, "...the best group effort ever achieved by Guachos," there's the temptation to toss the comment aside as mere hyperbole designed to generate some hype over a new release. But pianist/vocalist Guillermo Klein isn't exaggerating when he speaks of Filtros, his first Los Guachos studio album since III (Sunnyside, 2002). When it comes to composition and orchestration, Filtros makes it abundantly clear that Klein is every bit as significant as the more highly visible Maria Schneider and Vince Mendoza.

Since III, the Argentinean-born composer/bandleader released the remarkable Una Nave (Sunnyside, 2005). Recorded with an all-Argentinean ensemble, it represented fusion in the truest sense of the word—not the jazz/rock electric variety, but a seamless stylistic cross-pollination, where classicism and funk, freedom and form found distinct common ground.

With a slightly reduced configuration than III—eleven pieces instead of thirteen, and replacing a second drummer, singer and double-bassist with a trombonist—the similarly amalgamating Filtros doesn't break any specific new ground. Instead, Klein evolves his distinct mix of jazz, Argentinean folk music and European classicism with more expansive harmonies. Filtros' suite-like set of ten compositions are overall lengthier; both more compositionally detailed and improv-ready than the many miniatures of III, creating a unified work that better capitalizes on the strengths of Los Guachos' multifaceted line-up of musicians from North, Central and South America.

What Filtros has over Una Nave is its more urban edge, creating a vivid contrast with Klein's sometimes breezy writing, the complexity of which is deceptively hidden in its easy-going accessibility. The album begins sparely, with only Klein's piano and voice; but the ensemble soon enters gently, drummer Jeff Ballard, percussionist Richard Nant and bassist Fernando Huergo gradually stoking the fire beneath Chris Cheek's rugged baritone solo, leading into an equally evocative solo from altoist Miguel Zenon, with guitarist Ben Monder's chime-like accompaniment occasionally emerging above Klein's lush horn arrangement.

Surprisingly, the shifting bar lines of "Miula" do nothing to defeat its approachable veneer, Diego Urcola's muted trumpet and Zenon's flute an unorthodox but lovely combination for its buoyant melody. Underneath it's still a strong solo vehicle, trombonist Sandro Tomasi combining lyricism and a strong rhythmic complexion before Klein's arrangement reasserts itself for an unsettling passage of shifting tempo, an intro and outro to bass and percussion solos. "Manuel," on the other hand, is a simmering ballad, opening with contrapuntal horns alone, the ensemble entering to reveal that, underneath it all, it's really a blues of a different kind.

With ample solo space for Klein's outstanding ensemble, it's his unique ability to see confluence where others wouldn't that makes Filtros so compelling. "Vaca" combines an Argentinean traditional song with microtonal 20th Century composer György Ligeti's "Hungarian Rock," while the closing "Louange a L'eternite de Jesus" is an unusual look at Messiaen, a tone poem filled with dissonant harmonies yet strangely appealing. It's this out-of-the-box thinking that makes Klein so important yet underrated, and Filtros a high watermark of a small but consistently fine discography.