your computer input is too bad to record correctly your mix. Its not an issue of the U46' outputs.
Try this :
- mix with "real" vinyls
- connect the "rec out" of your mixer to the input of your U46
- connect the U46 to your computer and record with audacity or Vinyl ripper
The sound will be very very good.
Try the same operation by connecting your mixer to your computer' input directly ....
If you can't record your mix with Traktor (

), you have to buy a second audio interface to record correctly your mix.