I think you'd have to ask over on Head-fi to see if anybody has experience with those specific combinations.
The Xonar is advertised as being able to drive high impedance headphones and is measured at about 7V of output with 11 ohms of output impedance. That would honestly drive a very, very wide variety of headphones, and I wouldn't consider it necessary to buy 600 ohm headphones to pair with that.
For a tight budget, a used pair of HD-580s can often be found for not a ton of money.