neggy,
Don't worry about it!!! I feel the same way you do when I use the HD-600s through a soundcard or budget source, such as a stereo and its receiver!
There are two things to remember about these headphones. HD600s are very very sensitive to the source, amp, and cables, and are also by nature "relaxed" and "distance" the mids just a bit. They lack "bite" or "grind" in the high end as well.
I find them too laid back with my soundcard. They are grainy and weird sounding and I feel underpowered.
I believe you were misled: HD-600s are just decent with your kind of setup, but they get the raves from audiophiles that own a powerful amp and incredible source. For example, the headroom blockhead, a multi thousand dollar amp, makes them sound wonderful.
But I do think the other people who have posted are just correct. I'd buy Grados, or Beyerdynamics. I have had good experiences with both for the computer world.
I recommend!:
Grado SR-80
Beyerdynamic 250-80
No amp needed. Very wild sound. You will love them if you find the HD600s too laid back.
Cheers,
Geek