Very low volume on all inputs

First thing,,,,make sure the speaker wires are not touching each other.
i dont think it would be a fuse because that would cut the sound completely.you could open the unit to check for a fuse or fuses.
if none of the above help,then take the unit to a tech.i suspect a blown channel which means a capacitor needs to be replaced.one more thing...swap speakers over to make sure your speaker isnt blown