Finding pianist Fred Hersch listed as a sideman on someone else's album has much the same effect as ordering something in a restaurant and discovering it comes with champagne on the side. It's a treat, and it has a definite psychoactive effect: it makes the music sound better. Though he isn't so well known as some of his contemporaries (ex-Jazz Messengers Mulgrew Miller and James Williams, for instance) or even some of the next generation's wunderkinder (Jacky Terrasson, Marcus Roberts), Hersch's music belongs on the same shelf as all of theirs, and higher up than most. His solos sparkle, and only partly due to his pinpoint attack and powerful technique; much of their impact stems from tonal nuance, from the shape of his improvisations, and especially from the treasury of developmental ideas with which Hersch embroiders those shapes. (In these respects he shows his debt to Bill Evans, but with a creativity and originality that preclude mere imitation.) Certainly Hersch's keen interest in classical music plays a part--he's recorded two albums on which the compositions of turn-of-the-century French and Russian composers receive surprisingly successful jazz treatments--but for all its care and craft, his music avoids sounding cautious or fussy. This is no accident. Several years after revealing his sexual orientation, Hersch remains one of the few openly gay jazz musicians, and the only one I know of who has publicly revealed his HIV-positive status; I mention this not to generate sympathy or any of its less noble sister emotions, but because it indicates something about Hersch's risk-taking honesty and strong-willed perseverance, both of which are mirrored in his improvising. Those same qualities extend to the well-wrought arrangements he brings to well-known tunes--arrangements that require the practiced touch of two strong accompanists. Hersch has them in bassist Drew Gress and drummer Tom Rainey, both of whom will join him for this rare Chicago appearance. Saturday, 10 PM, Chopin Theater, 1541 W. Division; 235-2334.