I can't think of a more intrepid band of collaborators than the Ex. Already this year the Amsterdam-based quintet have toured with English saxophonist John Butcher, Congolese thumb-piano ensemble Konono, and French sound poet Anne-James Chaton. Over the years they've recorded with Sonic Youth, Tortoise, Kurdish folk singer Brader Musiki, Belgian comedians Kamagurka and Herr Seele, and too many Dutch free jazzers to list. But regardless of whom they share the stage or studio with, certain elements of the Ex's sound have been more or less constant. There's the anarchistic hectoring of G.W. Sok (aka Jos Kley), the sweet polyglot singing and martial rhythms of German-born percussionist-vocalist Katrin Bornefeld, and the dense, gnarled guitar playing of Terrie Hessels and Andy Moor (a veteran of Scotland's Dog Faced Hermans, he joined up in 1990). Bassist Luc Klaasen was equally integral to their sound until he left the band last year, but wisely, the Ex haven't tried to duplicate his signature sound slabs: new recruit Rozemarie Heggen's acoustic double bass reminds me of the rich, emotive cello lines that the late Tom Cora contributed to the albums And the Weathermen Shrug Their Shoulders and Scrabbling at the Lock. The Ex's two concerts here are their first in the U.S. since being turned back mid-flight on September 11, 2001. Thursday's performance will be a round-robin of improvisations with Italian ensemble Zu and locals Ken Vandermark and Hamid Drake. On Friday they'll play new songs they've developed since Heggen joined the band. Zu, Erase Errata, and Numbers are also on the bill. Thursday, September 4, 9:30 PM, Empty Bottle, 1035 N. Western; 773-276-3600. Friday, September 5, 9 PM, Abbey Pub, 3420 W. Grace; 773-478-4408.