If you know someone with an electronics background they could check/adjust the output voltage of the power supply using a 500W load. There are cheaper 24V 250W lamps around than the type used by De Vere. Simpy wire two test lamps in series to make a test load. Mount them where the heat will not cause any injury/damage.
I believe that the De Vere power supplies 'bleed' a little current to keep the bulbs warm to reduce the start-up stress? I have a single bulb De Vere head, with the lamp on measuring the lamp voltage shows 22V, Lamp off 1.45V. There are at least two types of De Vere power supplies, Linear and switchmode. Mine is a switchmode which appears to have the 'bleed' facility. Not sure if this is applicable to the Linear supplies?
Another possible cause is the ceramic lamp holders are burnt. They do arc when old and cause additional heat to the lamp.