The C111/C211 electrolytic caps measure as follows: 

C111: 2.03 uF, 6.98 ohm ESR @ 1KHz

C211: 1.94 uF, 8.27 ohm ESR @ 1KHz

Thanks for the info on the XJW01. I considered a DE5000 but that was a little beyond my budget (with the likely addition of import charges).

Found the problem...

Very much ashamed to admit it, but my previous statement that the bass/treble potentiometers measured ok turned out to be incorrect. The problem is the bass potentiometer: the resistive strip does not make contact with the right terminal (see arrow). When I did my original measurement I must have only measured resistance between the left terminal and the wiper (between R122 and R125 on the preamp board), not between the other terminals. As resistance was linear throughout the range, I concluded that there was no oxidation and the potentiometer could not be the cause. Such a stupid mistake...

Am I right in assuming that it wouldn't be feasible to repair the potentiometer as it cannot be soldered?

Clean it as good as you can.
Secure it using a small amount of strong 2-comp. glue and when fully set overpaint the joint using conductive paint/glue.
Alternatively, replace the potentiometer.

Martin

Thanks Martin! I'll take a stab at repairing the potentiometer using this method.