A couple of good options (actually probably better than what came in the unit originally) low cost replacement compression driver's might be Eminence or Selenium. I'm a little partial to the Seleniums for their reliability and sound VS cost. The Eminence is another bang for the buck choice.
http://www.parts-express.com/pe/showdet ... er=264-270
http://www.parts-express.com/pe/showdet ... er=290-446
These are the screw on variety, they also make the bolt on type.