Unless you are having a big problem with trying to match a +4 and -10 level together you should be able to just leave your soundcard volume up all the way. And, your mixer output should be at unity.
If the input to the soundcard is too hot in this case, you either have all your channel faders to high, and thus are probably distorting the output of the console anyway, or, you are running a +4 output from your console to a -10 input on your soundcard.
If you had to choose which to turn down, it should be the output of the mixer. Leave the input on the soundcard all the way up. You would be using software control on the computer to turn it down, and that would not offer the best fidelity because you would be changing the number of number's after the dicimal point in the bit length. On a 16 bit converter, this is death!!! It cannot do anything but "average" the extra numbers after the decimal point, and that will make your .wav file sound dull and edgy. ANY volume change you apply to a 16 bit file will do this. With 24 bit you have less worry, but eventually you have to dither the file for 16 bit playback.
Ed
[This message has been edited by sonusman (edited 03-09-2000).]