I'm not familiar with those exact headphones, but if they have a volume knob, make sure the headphone volume knob is max for detection.
You can potentially get better audio from the back of the case, but unless you're a pure audiophile or ran your cables through the worst places allowing noise induction, you probably won't notice the difference.
I think you can re-assign the audio jacks on the back to accomplish that, but it's just a guess and I have no experience doing so on these Asus boards. Plug some in and find out.