The duo of saxophonist Anthony Braxton and trombonist George Lewis contains at its heart an exciting purity. This concerns more than the minimalist instrumentation. It has to do with the unique voices of the two instrumentalists--even though, at this point in their careers, both have established their primary reputations as composer-theorists (to the point of sometimes obscuring their achievements as horn men). Many noteworthy jazz soloists have presented a distinctive syntax in their playing. But relatively few have managed to add a new vocabulary, as have both these players (longtime members of Chicago's Association for the Advancement of Creative Musicians). Braxton's fiercely articulated, relentlessly nonscalar approach to the high saxophones, combined with his remarkable control of subdivided rhythms and timbral effects, have led him to a style that sounds like no one else's; it is a technique so striking that it will probably influence even those saxophonists who reject the aesthetic frame of Braxton's music. Lewis, building on the startling 1960s technical advances of the German trombonist Albert Mangelsdorff, became the AACM's J.J. Johnson: just as Johnson became the first trombonist to apply the full flower of bebop to his instrument, Lewis's precise mastery of extended technique gave the AACM its first world-class trombone soloist, on a par with any of the saxophonists who came to symbolize the organization. The relationship between Braxton and Lewis dates back nearly two decades, to Lewis's appearance in Braxton's quartet at the 1976 Berlin Jazz Festival, and no one would question that Lewis's early mastery of Braxton's rigorous and ingenious small-group writing helped shape his own ideas on composition. But this concert places the prime emphasis on the remarkable performance praxis each brings to such compositions. Part of the AACM's 30th anniversary weekend, Braxton and Lewis will cap an evening that also includes the Ethnic Heritage Ensemble, the band led by Steve and Iqua Colson, and the New Horizons Ensemble. Saturday, 8 PM, Getz Theater, 72 E. 11th; 752-2212.