Hi,
much good advice here, except I doubt it to be the amp. I've got one of these myself and send it 6V from my dCS stack, and nothing problematic about that. I would still try to isolate everything step by step, as suggested, but the likely culprint is the computer transport. Try play some files with Amarra, Pure Music or the like instead and see wheher that solves the problem.
I use Win7 and can only play with JPlay, all other software introduces distortion.
Best wishes
S