maybe try installing previous drivers like 8.0.8.i got also a problem within the installation ''unknown error'' with the 8.1.8 drivers,including the uni 1.72v drivers as well.the uni 1.71 however works.
asus dropped their support and crapped all over the asus xonar dx unfortunetly drivers wise.there is not much to be done about it 😞
edit:i wrote that in mind u had xonar dx,as for phoebus i have no clue