Just to check:
1./ Check your soundcard and make sure that all treble/bass/aural exciters are set to ZERO
2./ Make sure the impedence setting is correct on the soundcard (if you have that option)
3./ Set to stereo headphones, NOT, anything else whatsoever
4./ Make sure and EAD/filters etc. are OFF
Remember that your ears got used to your old set up and take a while to "retune" themselves. You could also try running your soundcard into a hi-fi amplifier and then drive your headphones from the amp. It is sounds very different, then your soundcard set up is all wrong or CANT drive your headphones properly.