One suggestion to get you by is to get the Creative X-Fi Go Pro. It's $35 online and is an amp/dac combo. It's pretty much ruler flat I think and sounds pretty impressive for the price. It actually drives my HD-580 quite decently!
You can even get them at Best Buy (can check stock online) and then return it if you don't like it. It's cheaper online though.
I remember loving the HD-598 with this tiny DAC. Not sure why it sounds so good.
There's some measurements on another site (that I can't name) and it did pretty well.