Bassist Ken Haebich and saxist Jim Gailloreto, both veterans of several musical demimondes–mainstream Chicago jazz, classical chamber composition, and of course the wedding circuit–have come up with a flexible format for this concert series, capitalizing on the variety of their experience without drawing directly on any of it. For a few months now they’ve appeared every Sunday at Nevin’s Live, a surprisingly commodious performance space connected to an Evanston restaurant, and each time they’ve invited a surprising combination of guest artists to join them in a night of extremely impromptu music. The groupings of guests often display a certain witty perversity: Last month, for instance, Haebich and Gailloreto hosted Tim Mulvenna, who drums for freedom-loving firebrands the Vandermark 5, and Fred Simon, a lyrical pianist in the ECM tradition. And two weeks ago they brought together virtuosic jazz-rock drummer Larry Beers, keyboardist Jim Baker–probably best known for his spacey collaborations with north-side avant-gardists like Mars Williams, Hamid Drake, and Guillermo Gregorio–and gifted operatic soprano Winifred Brown. With such a potentially unstable mix of personnel, the band shouldn’t even have made it through one set; the fact that it proved wholly listenable and often fascinating testifies to the leaders’ ability to create a comfortable, open frame for their collaborators. In such a setting, the guest musicians sometimes find unexpected connections of their own: a trio of Brown, Beers, and Baker provided one of the strangest and most affecting pieces of the evening, referencing Baroque music as well as serialism without sounding academic. Haebich plays both stand-up and electric bass with cool fervor and a terrific ear for harmonies; Gailloreto’s solos often have a neat, inexorable quality, as if they could only have unspooled the way they did. And together they’ve come up with the closest thing to a regular “happening” since such events proliferated on the coasts in the 60s: a minimally orchestrated, anything-goes musical grab bag, where the chance to hear a genuinely unprecedented collaboration outweighs the risk of a dud. This Sunday, August 12, the Ken & Jim Show will feature Indian percussionist Kalyan Pathak; August 19, guitarist Dave Onderdonk; and August 26, percussion powerhouse Paul Wertico and the creator of “word jazz,” Ken Nordine. Sundays, 8 PM, Nevin’s Live, 1450 Sherman, Evanston; 847-869-0450.