You simply want to include the 802 in the loop from the soundcard line-out to the soundcard line-in when you do the calibration. When you save the soundcard cal file, the response anomolies will be in there, along with the soundcards response anaomolies.
That way, the response of the mixer is compensated for and won't effect the measurements.
Here's a pic of my 802 with the two cables connected. The gray cable on the left comes from the soundcard line-out and the red cable on the right goes to the soundcard line-in. You have to play around a bit with the levels, but no big deal.
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Make sense?
brucek