You say you're looking for a musical experience that's really--despite overuse of the word--unique? You say you really do give a hoot about that fabled Rubicon between the jazz and classical avant-gardes? And you want to actually enjoy the occasion, too? Then you'd be crazy to miss this weekend's performances by ROVA, the San Francisco-based saxophone quartet. Combining the shock of the new with the traditional sonorities of a finely tuned sax section, ROVA can be counted on to try almost anything, and to make it sound good in the process. (In fact, the quartet's most innovative music has a deceptive familiarity: it welcomes rather than repels unsuspecting listeners.) At the age of 13, ROVA still places more emphasis on composition than many of the sax ensembles that have followed in its wake--which is not to deny the committed and purposeful improvisation that also distinguishes the group. This is music making of a very high order. As is often true of such music, there's a rich if sly humor lurking around the edges; it should be especially evident in Sunday's performance, when ROVA will improvisationally "interact" with the booming acoustics of the neo-Gothic Bergman Gallery, and also with the Niele Toroni installation currently on display there. Saturday, 8 PM, Southend Musicworks, 1313 S. Wabash; 939-2848. Sunday, 4 PM, Renaissance Society, Cobb Hall, University of Chicago, 5811 S. Ellis; 702-8670.