[Arran]

I think that Mike meant the filter cap at the input of the field coil not output, since if the speaker was unplugged you would not get anything at the output. Again you have to be careful what you use as a ground reference point, this set has a floating negative side, true B- is the center tap of the power transformer with a chain of resistors in series with it. You will want to check the resistance across each capacitor from one side to the next, not from one side to the chassis ground unless the other side is connected to the chassis. But if you get a resistance reading check to make sure the cap is not connected in parallel with something like a coil or a resistor, if it is clip one lead on the suspected cap and check again.
Regards
Arran