I agree with Scott that you need to check the quality of the headphone driver on your PC before buying expensive headphones. I have (lowly) Sennheiser HD535s and they sound COMPLETELY different on my PC, my portable CD and direct from the headphone jack on my Marantz CD67SE. The Cd67 sound is leagues above the portable CDP, which in turn is a more modest step up from the PC.
The headphones may not be the weakest link ... you may do better buying a new source (even a standalone CDP with a variable volume headphone jack like my Marantz).
The headphones may not be the weakest link ... you may do better buying a new source (even a standalone CDP with a variable volume headphone jack like my Marantz).