Looking for new soundcard

The Sennheiser HD598's are a good headset, and you have a few options to go to. Knowing a price range you would like to spend would help a bit...

For standard analog hookup, I'd recommend either the ASUS Xonar D1/DX, HT Omega Claro+ or Auzentech Meridian 2G. Comparable cards, with a few differences between them.

If you think the HD598's are a bit low volumed as is though, then you might want a soundcard with a powered amp, making your primary options:

ASUS Xonar DS, Auzentech Forte, ASUS Xonar Xense, ASUS Essence ST/STX.