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.
150 ohms + means it's pumping out the max volume which is used for anything above 150 impedance. It's doing what it is supposed to do. The back panel will not provide better sound quality and may even reduce the volume you get through your headphones as it's designed for speakers which have much lower impedance than headphones. Use the front panel for your headphones or get an external DAC if want better quality audio.