Out of interest did the electrician check the line while your equipment was on and running, drawing current. Perhaps the equipment is drawing a large amount of current (maybe even due a fault in the equipment itself, which could be dangerous) and this is causing the voltage drop. The electrician should be able to measure the voltage at the B&K while it is running. This would prove for sure if it is low voltage causing the relays to operate, and if it is, whether the equipment loading is causing the low voltage.
I find it hard to believe that a HiFi can pull enough current to drop voltage, but it's worth eliminating that possibility (and the possibility that the voltage isn't low and the B&K is defective) before spending money which might not fix the problem.
I find it hard to believe that a HiFi can pull enough current to drop voltage, but it's worth eliminating that possibility (and the possibility that the voltage isn't low and the B&K is defective) before spending money which might not fix the problem.