If you connect a preamp or your head preamp to the insert loop, note that in all factory presets, the insert loop is set to OFF by default.

To activate the loop, press EDIT and go to the LOOP meny with the PAGE encoder.

Set the INSERT LOOP to ON, Then press MENU and go to the UTILITY menu. In this menu, set the parameter INSERT LOCK to 'ON'

You can also visit this page from TC Support to view a .pdf file showing a diagram explaining all connections.
tcsupport.custhelp.com