The hum could be from electronic interference, or it could be a loose connection or failing component in the power supply--does the hum occur when he uses battery power? If not, try using the power supply in a darkened room--lights, television, radios, alarm clocks, cellular phones, pagers etc. turned off--then turn in a full circle and see if the hum is still there or if it disappears while you turn. If it remains, it's probably the power supply and can be repaired or replaced with a newer power supply, but the bass sounds like a candidate for the electronics upgrade. The upgrade costs $1000 to $1500 US depending on the age of the bass, which will have to spend a few months at Alembic's shop, but is said to screen out almost all forms of interference and make the bass absolutely silent.
In any case, 2000 Euros for that bass sounds like a deal. I'd say go for it.