When connecting the Tiamat 7.1 you need to think of them like a 7.1 speaker set and not a headset. You connect them like a 7.1 setup and have to setup your software as if a 7.1 speaker setup.
Your issue is your sound card only supports up to 5.1, how you get around this I don't know. Is there anything in the Tiamat's instructions.
As for sound quality the Tiamat's need a high quality sound card. When I first got mine I was very disappointed in the sound quality but after reading reviews it became clear I needed a better soundcard. I ended up going for the ASUS Pheobus which was recommended in some reviews and this made a huge improvement. Apparently onboard and low end sound cards just don't have the power on their 7.1 outputs to...