Take a close look at those pots where the brown insulator board leaves the metal casing, and check carefully for any cracks that might have broken the circuit through the pot. This kind of damage is quite common if mechanical stress is put on the pot.
Also, check carefully all the soldering and the traces (the copper paths) on the circuit board around the pots and the connectors - breaks in these areas are common, too, because of stress and vibration.
If you're lucky it may just be a "mechanical" failure of this kind. Could be bad caps though, of course...
Could you better describe the sound that it makes when no source is connected (to an engineer, "hum" and "noise" are two very different things!)? How does it perform with a source connected?
Good luck, and kudos for having a go at it!