Uninstall and take the TV card out. With the TV card out, hook up the amp to the sound card. If it sounds good then there's a problem with the card, either it's faulty or causing a conflict. Possibly, connecting the TV card's line out to your soundcard's line in could be causing the problem.
I would recommend to anybody that works with audio, whether it's recording LPs to the HD or using the PC as a recording studio, to dispense with the onboard sound and buy a soundcard. You should be able to buy a Soundblaster card for under £20.