http://warzone.org/soundcard.html (http://warzone.org/soundcard.html)
Ouch...
Maybe it's just a problem with a low quality 'line in'. Try using the internal loopback.
http://warzone.org/soundcard2.html (http://warzone.org/soundcard2.html)
Better.. what would make total harmonic distortion and intermodulation distortion so bad?
What OS are you using for testing?
edit: Did you adjust the volume control at RightMark Audio?