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Hum usually indicates a power supply problem. If it has an internal supply you will need someone to check it's producing clean DC. Alternatively if you do open it (beware risk of shock!!) initially visually check condition of /replace any bulging electrolytic capacitors.
It could be the diodes, but whatever is the cause it's before the pre-amp section. Generally a loud hum such as that with no effect by volume control, is mostly caused by the filter capacitor in the power supply. And that hum is the 50HZ mains hum getting into your system!
Sounds like the hum might be mains hum caused by the failure of the power supply capacitor(s) in the sub. It/they will be the largest on the pc board. You will find all the details (needed to replace it) on them. If you find more than one replace both.