Sound Card with External DAC

I'm putting together a high end stereo system that will be connected to my desktop computer. I hope to connect the computer to a high end (separate) audio tube DAC. The computer and its transport will serve as the source component, and the tube DAC as the converter. Any ideas how to best do this.
If I understand correctly, standard sound cards already have a DAC built-in. I don't want to be redundant. Is there a sound card that will allow this? Thanks. Jim