I have mine set to 8ch right now, but will try 6. Haven't noticed any problems so far, but not had a proper play around with the settings.
But tried a few games, and holy ******* ****! I'm amazed at the quality. I thought the Medusa 5.1's I have were alright, but wow they were awful. Im hearing sounds I never noticed before and virtual 3d is so much more effect than 5.1 in headphones.
I believe the soundcard amp isn't quite powerful enough to power the NS1000 headphones. Although I think batteries in the headphones does a similar job, they just need replaced constantly. One thing I have noticed is at the moment the sound is a little echoy (cant think of better term). but that could be because I am used to Medussas which were so flat.