Are the readings with one DVM or are you relying on a separate meter in the back? Just want to make sure its from one meter.
Assuming its one meter - Check the voltage at the input side of the rear inline fuse holder, then on the output side you should be able to stick your probe in there to get a readying without taking the holder apart. Then compare these reading to the rear batter voltage and the amp voltage. If you see a major drop on the output side of the holder, you might want to check the connections of the fuse and the wire going in and out. MAKE SURE you keep the ground probe at the same location for these voltage readings!!!