ive got the normal gax coilovers with 300lbs springs on the rear and found that they didnt raise the car enough so i sent them back to them to get them extended which they done

now having them on lower setting for road use, when the shock opens up, the spring is loose so on big bumps there is a clonk when going over bumps on road.
if i turn the damping knob around 3 full turns, it doesnt bounce as much and stopped the shock opening up too easily. Obviously youll loose a bit of ride comfort but nothing you can do christian.

try do what tim said tho, should keep the spring tighter in there and sort out the prob.

IF you still not happy, just call Gaz and they will sort it for you