Don't know the specs but i do have Beyerdynamic DT 990 Pro 250 Ohm headphones and the Z97 pro wifi-ac. The sound via the asus board is outstanding. I am fairly certain the board drives the headphones just right. Compared to my two Onkyo receivers (TX-SR503E and the newer TX-NR636 (dolby atmos) the sound on the asus is way more balanced and sweet. On the onkyo's the DT 990 pros have really harsh highs, to the point its really unpleasant, not so on the asus! Do make sure your source material is good though, its now easy to tell difference between an mp3 and a FLAC file.
I was considering and external DAC and headphone amp, but i very much doubt the sound would get (audibly) better. The volume is easily sufficient as well. I typically listen at 18% volume. 30% for prolonged use would give hearing loss I think, they go really loud without distorting.