It may be the ASUS drivers. A third party driver for Xonar and related cards claims to greatly reduce DPC latency:http://maxedtech.com/category/uni-xonar/
Uninstall the old ASUS drivers and do a clean install with these.
As far as I'm aware those drivers don't support the Phoebus?
EDIT: They may support it, but I don't think those drivers come with the same control panel that I need to set up my 7.1 and fine tune things for my headset.
EDIT2: A few weeks ago I had a Xonar Essence STX (forgot to mention this), and I tried the UNi drivers but my latency was still the same.