The basic KEF 2000XXX subwoofer series suffers from many poor long term design problems. These can be sorted bu any competent bod in less than one hour.
Some capacitors are cooked by hot resistor voltage droppers, moving and replacing a few transistors onto the heatsink with better specified types. The job is messy but quite easy. Unlike my cursed MJ Pro1 MKII which I am having to repair for similar reasons for the 3rd time! The KEF mod seems to work just fine...