Same resolution, same refresh rate?
If it's the same monitor staying less bright no matter what port its using, then it's the monitor. Pump up the brightness maybe, but monitors should be virtually identical from the factory, especially monitors of that caliber. BTW, it looks like an uneven backlight, from the picture. The right side is brighter than the left side, or just the camera.
Also check on Dell support forums. I'm sure you wouldn't be the first having dual 2311H coupled with a ATI card.
In the end, call the supplier (OcUK?) to see if you need a RMA.