I believe that the controls are 50 ohm 3 watt wire wound pots, not 'L' pads. If after spraying them internally with a contact cleaner and finding them to be still not working then it is likely that they have gone open cct. You can fit an 8 ohm ' L' pad in place of the pot by just adding an 8 ohm wire wound 5-10 watt resistor to the negative speaker connection . With most 'L' pads the positive input goes to terminal 3, the centre tab of the pad, terminal 2 goes to the speaker +ve. Terminal 1 is where the resistor would be added, in series to the negative speaker line. It is not a perfect constant impedance solution but then the Wharfedale crossover circuits are typical for their era and far from modern high end design approaches.