Hey,
 
I have the Asrock Fatal1ty z170 Gaming K6
 
It supposedly has decent on board audio which at least to me sounds rather good quality and more volume than I could ever want.
 
I plug my DT990 Premium 600 Ohm directly into the front headphone jack on my PC - I love my DT990 there a little bright but what headphone is perfect!?? I'm very happy with these.
 
I remember Dolby Headphone supposedly making stereo headphones sounds sort of like 5.1 which would be obviously better for positional audio in gaming.
 
May I ask as things stand do I have any options available to me which won't break the bank with regards to keeping audio fidelity and using my DT990 but getting better positional audio in games?
 
Much appreciated.

I'm not familiar with the driver/software package for that board - does it not include Dolby Headphone or similar?

Anyways, you can add the Sound Blaster SBX Software pack (which brings SBX) as long as the audio device has an HD-codec, which the Z170 will:
http://software.store.creative.com/p/software/sound-blaster-x-fi-mb3

What obobskivich said works. If you want a hardware solution, I think probably the soundblaster z is the cheapest. It doesn't have dolby headphone, but creative's version of it.