If I am not mistaken, tape outs on a master section almost always come post fade, so the main fader level would certainly affect them. Not only that, but if they are on their own pot, that means that the signal runs through even more circuitry, which in a Behringer is probably not the greatest idea. Then there is the whole balanced versus unbalanced thing. In the original posters case though I believe that is somewhat of a moot point since it sounds like the sound card inouts are unbalanced anyhow.