Have just repaired a similar problem with a KEF 2500 - relay was chattering at 50 Hz rate, buzz in speaker output.
Problem was caused by overheated / dried out Electrolytic filter cap (C52 - 47uF 63V) on the relay coil supply feed.
The overheating is caused by adjacent power supply dropping resistors which run very hot due poor design of power supply.
A service manual for a similar model is available at following link, refer to power supply section:-
1) remove chassis from speaker cabinet (8 screws), disconnect red/black speaker wires from main PCB spade connectors.
2) Remove 4 screws attaching main pcb heatsink assembly from face panel - lift assembly from faceplate (a bit messy , much heartsink compound between mounting faces)
3) unplug the AC 3 pin connections adjacent to the large capacitors and the 3pin connector on end of pcb (near C52), then roll assembly over the top of the audio sub-board to access underside of pcb.
4) solder connections to C52 is accessible at edge of board without further dis-assembly.
Take extreme care replacing C52, as the circuit board is of poor quality, (usual domestic electronics quality) being single sided copper without plated through holes.
The board is already heat damaged by heat buildup from power supply resistors, so the track will most likely lift and crack when disturbed- to counter this problem link the +ve capacitor pigtail to an adjacent solder pad on the same track.
Assembly is reverse of above, take care to preserve/ensure minimum disturbance of heatsink compound.
A user tip- don't leave unit switched on when not in use- heat buildup will eventualy cause it to fail due dried out electrolytic capacitors.
Poor power supply design is the main source of the standing heat buildup.