I have a Sennheiser HD650 and Asus Xonar U7 with me purchased very recently.
Since HD650's jack is a 6.35mm one I had to get an adapter jack and I purchasede something from your typical local electronics shop, which looks and is cheap -no fancy stuff.
The adapter's female part is a bit loose -the sound may go mono -as in only L or R- if I touch the joining part- it's that loose.
HD650 is far from being burnt in -so maybe it has something to do with it- or Xonar U7 is not the right choice to feed those cans however I noticed trebles "drowning" at max levels -as in they mix into each other- same thing you'd hear if you'd turn your common TV's volume too much.
So, maybe that adapter jack is eating up too much in between the fancy gadgets it's connecting? Could it be? If not, what else ?