There are three reasons the Chicago Pro Musica is one of the most dynamic, versatile, and progressive chamber ensembles in the world today. One is that all but one of its members belong to that elite of instrumentalists--the Chicago Symphony--and the one exception, Easley Blackwood, is no slouch, being a keyboard wiz and iconoclast composer from the University of Chicago. The second is they perform for the sake of performing; they play whatever strikes their fancy. Which, thanks in part to the stewardship of ace clarinetist John Bruce, has meant a preponderance of near classics of our century and challenging, unfamiliar pieces from earlier eras. Thirdly, their Grammy-winning albums are among the best produced I've ever heard: gorgeous sound, masterful engineering, and definitive interpretations. Need I say more? Because of their CSO and other duties, the CPM players seldom get together for concerts. Here's your chance to hear them live, performing, among others, Charles Wuorinen's Bearbeitungen uber das Glogauer Liederbuch (1962), Donald Erb's Quintet (1976), and the Bassoon Quartet of Mozart (arranged by Willard Elliot). One added attraction: $6 admission. Sunday, June 7, Orchestra Hall, 220 S. Michigan; 435-8111.