Are you gamma-correcting this at all? Easiest way to do this is to use an sRGB backbuffer/RTV. You'll also need to degamma any color stuff to counteract the darkening effect.
EDIT: A (0.5/255) bias alternating across pixels/frames will also help somewhat. The idea is that you shift brightness for pixels that sit in between the two displayed steps of color so your eye will do a little bit of extra temporal integration as they flip-flop over time.