Wow, you guys have some awesome setups, it's pretty intimidating coming in here with such dedicated audiophiles!
Anyway, I decided to upgrade from some crappy Razer headphones (they had a constant ringing sound for some reason), and to also ditch my Realtek onboard sound. I got:
Sennheiser PC 350:
Asus Xonar D1:
In comparison the audio is MUCH clearer and sounds excellent. I'm hearing sounds in my songs I've never heard before, it's kind of scary when you're not used to it!
When I posted these two purchases on a different thread, someone highlighted "Sennheiser PC 350" and said "/facepalm". Is it really that bad of a pair of headphones? Compared to all the headsets I've owned, this one sounds the clearest, but I'm sure the Xonar is helping that out.