Assuming the input to the 518 is not already pre-emphasised (the red EMPH light will show this), using EMPH will ensure the 518 output is ALWAYS pre-emphasised and will give you audible gains in your system (be careful of material that has too much high-frequency signal though)
Try a well recorded track and relax and listen several times first with the shaper FLAT, then HP, then each SHAPE A, B,etc....
Although Shape A and E have very similar curves they have different degrees of noise reduction so try switching between those two after your initial test above. It takes time to learn and get used to the 518 but it is worth the wait..!
Regards, Richard.