have you inspected them to be sure they're setup properly to begin with?

one trick with the avids, or other conical washered brakes, is moving the fatter of the two concave washers to the inside of the brake, which will adjust the angle the straddle cable sits by widening the stance of it. you could also try adjusting the straddle cable height itself... which means you'd have to ditch the avid fixed length ones, if that's what they're setup with. i was able to get better braking performance from my wife's tektro oryx's by reconfiguring those washers and shortening the straddle height. you could also try better brake pads. salmon koolstops come to mind.