I have a not so great set up currently, just a pc with an Asus Xonar Stx sound card and the TH900 (which came today !)
I noticed, when i put on the headphone, I had it on speaker, then i switched the soundcard to heaphone mode, I can hear this little hiss.
I don't think I noticed this hiss with the Beyerdynamic MMX300 headset..
Is it the sound card or the headphone? ( I hope soundcard)
I would say it's the sound card. Have you tried the TH900 on other devices like iPod or even your motherboards on board sound card, also make sure your computer is properly grounded. If the hiss remains try a process of elimination, start off by unplugging everything that's attached to your PC and even move the sound card to another slot. hopefully this helps, let us know how you go :)