Perhaps the most versatile and least doctrinaire of all the midsize groups devoted to promoting contemporary music, the Frankfurt-based Ensemble Modern is embarking on its first U.S. tour, with a cutting-edge program whose title, "From Adams to Zappa," identifies two of its seminal collaborators. The Bay Area minimalist John Adams began working with the 20-musician ensemble--which was founded in 1980 under the auspices of the German Youth Orchestra--about three years ago when he attended its recording sessions in Los Angeles for Frank Zappa's The Yellow Shark CD (on Barking Pumpkin), an assortment of the rock innovator's forays into "serious" music. Since then Adams has written several compositions tailored to the virtuosity and stylistic flexibility of the EM. Two are included on the program: the gleefully rowdy and impudent Chamber Symphony--whose third movement, "Roadrunner," is an homage to the cartoon trickster--and Scratchband, a rock-classical hybrid to be performed for the first time during the tour. With these hyperkinetic, pop-influenced pieces, Adams continues the legacy of Zappa, whose irreverent wit and odd instrumentation owe a debt to Edgard Varese. Not surprisingly, Varese--whose complete chamber-orchestra works have been recorded by EM under Zappa's direction (to be released later this year)--is represented on the program by Octandre, a marvel of energetic syncopations. Other selections also reflect the EM's taste for gifted American eccentrics. The three Studies for Player Piano by the reclusive elder iconoclast Conlon Nancarrow consist of intricately woven rhythmic layers. And the three tracks from The Yellow Shark are charmingly mischievous, Varesian fruitions of Zappa's experiments on the music computer Synclavier. Adams is the guest conductor, and he should make the most of the ensemble's precise, crisp, spirited playing. Monday, 7:30 PM, Orchestra Hall, 220 S. Michigan; 663-1628 or 435-6666.