Hi Stew,
Not sure what your input is? or what is in the FX loop. Typically there is plenty of gain with the LO gain setting, and HI gain setting is used for Piezo pickups or Ribbon mics.
Even when the red headroom comes on it mostly signaling that soft-clipping ckt has been engaged with a couple dB of headroom left before you hear a distorted signal.
A simple test for unity gain between FX Loop active vs FX Loop not active is to just use a patch cable between the To FX and From FX jacks. Currently what are you using in the FX loop that you don't quite see unity gain. Also the tail/no-tails setting may have small effect on gain as it is muting the dry signal.
Also, please email me a diagram of your setup and I will follow up in more detail.
thanks,
