iMac audio issues

I have an iMac (model right before the first dual core) and have been running through an M-Audio Audiophile firewire eternal DAC for about four years. During this time, the internal iMac speakers by and large didn't work when selected in system preferences.

I now have an HRT DAC I would like to compare to the M-Audio DAC, and I can't get a signal to it. The HRT shows up in the audio preference list, but the only sound I can get through the iMac to the HRT is sound effects. I can also get sound effects when the iMac internal speakers are chosen.

I have disconnected the M-Audio and deleted all related software with no improvement. I think I have a software issue that needs to be repaired.

Anyone else deal with iMac audio issues.