Hi Martin,

The SR5 and in fact all my power supplies unless otherwise requested have a floating negative return. This avoids multiple ground loops with multiple power supplies all connected to the same system, thus avoiding ground loop hum. The power supply circuitry takes it’s ground reference from the equipment it powers. The power supply chassis are all earthed to mains earth for safety. The third wire in the Power supply DC lead is connected to the power supply chassis and therefore mains earth but this may not be the optimum path for an earth connection with some equipment, so I leave this wire floating.

You have already found a cure for the ebony board insulation problem by earthing the deck to the Whest preamp earth post. This is probably the optimum earth point, as it will hold the deck chassis at close to the earth potential of the preamp chassis without traversing a polluted system ground.

Regards
Paul