Rather than a soundcard per say, which seem to introduce a fair amount of jitter by their nature, you may wish to consider a Sutherland dax7. In its stock format, it is digital in - analog out and a volume control. It uses 12ax tubes in the output stage and is great for going form digital broadcasts on line into a main system. It was a huge upgrade in quality and opened all the digital radio stations into listenability. My version was highly modified, so the are flexible as to your nsystems needs and impedances. stock unit is $1600.00. Don't know if it is on the used market yet.
good luck
cbird
good luck
cbird