There is power at the plug that then runs to the fuse then out the fuse through a diode (to protect against reversed polarity) then to the power supply.
Here's a thought. Check the power from the fuse leg to the diode. Then check the diode. If both are fine, consider removing the amp and use a piece of 1/2 in MDF to seal the back of the tube. Power the sub with an external amp that has 100 - 200watts RMS output. It may surprise you.