Depends on what the output of your Mic Pre is. Check the specifications of the AudioBox and your Mic Pre to make sure you're matching levels and there isn't an impedance mismatch. Then you'll know whether to use Mic or Instrument. If you're still not sure, contact your Preamp maker to see what they recommend.
Above all, you will want to make sure that the preamp levels on the AudioBox USB are hard left (-10/0) so there is NO gain (second gain stage) on the Mic or Instrument pre-amp (which ever one you end up going with).