Can I bypass the preamps in Duet 2 (USB)?

YES!  Duet 2's preamps can be bypassed if you'd like to connect a line level input source such as an external microphone preamp or mixer.

Here are the two things you'll need to do:

1. Connect the devices line level output to Duet's combination input jack using an XLR cable.
2. Go to the input tab in Maestro (found in your Applications folder) and change the input selection from "Mic" or "Inst" to "+4 dBu" or "-10 dBV".

The line level selection will depend on the type of gear you're connecting.  Professional audio equipment most often uses +4 dBu, while consumer audio gear is set to -10 dBV.