I'm using an M-Aduio Audiophile 192 sound card and run a coax digital out of the PC to an external DAC and to my amps. It just sounds flat to me
You should avoid any D to A and A to D conversion. Clearly by sending things through your sound card then you are making a conversion somewhere in the PC. Clipping in an A to D might explain what you describe.
If you are not making a D to A and A to D conversion then the sound cannot change between a CD played in your PC or from a separate CD transport.
Another possibility is that their is a bug in your compression and de-compression steps that introduces losses.