When I connect my expression pedal to the Nova System, the volume automatically resets itself to -100dB.
The solution for this is to re-calibrate the expression pedal. This reset happens when the pedal is disengaged during calibration or otherwise, and it is meant to prevent any damage that could be caused by the Nova System going to 0dB.
If you are not using an expression pedal to control the volume on the Nova System, please go into the Pedal menu and make sure that the parameter called "Type" is not set to "Exp. GlbVol" ("expression pedal, global volume"), but simply set it to "Expression" instead.