Here's a very suitable -30dB attenuator that will lower line to mic level from unbalanced to a balanced XLR mic.I think I can make the 100:1 pad
I drew it for you in Visio below. Very simple.
Hook it up and create a soundcard cal file and save it.
Then measure to be sure of your flat line.
Remove the loopback and use that channel for measuring.
When you're done, tell us how much different the soundcard file is from the previous line-out to line-in file.