Those are nice headphones. I have a pair. I also have an Asus Xonar Essence STX. The quality improvement over my mobo codec is a moderate amount, generally flatter sound which will be closer to the intended audio output. Your codec is the Realtek ALC1150, which is already good, but you may find an improvement with a good headset (HD 518's) and decent sound card. One of my reasons for getting the card was to reduce recording latency, which it does also. I would suggest you buy the Headphones first and then decide whether or not there is room for improvement or if you really need an improvement as on board audio these days tends to be quite good, as long as you don't need to record professional quality sound or have a real ear for perfection.