Thanks for all the help, guys. Seems like I might go with DT770 80 Ohm HE-400 is a bit pricey for me here in Norway. While it goes for the manageable sum of $299 on most US sites, it goes for around $499 over here.

If I were to buy one of the beyerdynamic headphones you listed, which one would best suit my needs?  Judging from this guide, it would seem that the choice is between DT 770 PRO 80 Ohm, DT 770 PRO 250 Ohm and DT 880 PRO 250 Ohm. You say a 50/60-Ohm headphone would do "decently" well, does...

I'm using a Creative X-Fi HD (link), which I suspected could be the culprit, but after googling a bit, I saw that it did receive some praise in a few threads on this forum (aswell as some criticism, but not to the extent that I could conclude that the sound quality my headphones output was due...

I've had the Beyerdynamic Custom One Pro for about a year now, and it's been an up and down experience. I decided to buy it after my old DT 770 PRO broke, and I read on various internet forums that COP was similar to DT 770, only with stronger bass. The stronger bass part is certainly correct...