Your amp's setup guide should provide the best way to adjust the loop. However, you'll typically want to set the Send as high as possible and adjust the Return to set unity gain. With your amp at relatively low volume with the loop deactivated, starting with the Send and Return all the way down, engage the loop and start playing (strumming the strings open works fine). Increase the send level until the IN 2 light on the FX8 indicates clipping then back off slightly (probably won't hear anything since the Return is off). Adjust the Return level toggling between the loop on/off to achieve unity gain. Incrementally increase the volume of the amp to your gig or preferred level and make any further adjustments as necessary.
