What is the reading to earth at the board without the transducers connected Mark? Given there is an earthed contact on the transducers, it seems likely to me that removing the basket with the circuit active may have caused an earth loop somewhere. You may have come across an earth loop sometime in audio equipment where too many earths can cause a loud hum in the system. An earth loop can cause quite high currents to flow which may have blown the fuse/board components.
