I had the same thing happen once with my Dewalt miter saw. I tried clamping to the bench as you have done, but the fence would spring back. I shimmed up the ends of the fence about 1/2 inch above the bench and clamped the center beyond the point where it was straight. When the clamp was released, it was much closer. A couple of more tries, and I got it perfectly straight. I'm very much more careful with round stock now!