@jea48 is correct. When this amp was built it had an ungrounded power cord. Someone replaced it and now the amp is grounded to the building electrical ground (per EU Directives).
The trick is to have the audio ground of either the amp or preamp be separate from the chassis ground, with a resistance between the two that allows the audio ground to float at chassis potential, yet is a high enough value to prevent significant ground loop currents. 100 Ohms is plenty high enough! But because the amp is using the chassis as ground you'd have to find all the places the circuit is grounded to the chassis, this kind of repair (since this really should have been done when the amp was built) might be tricky. A technician might be able to look at it and let you know.
Plan B: get an isolation transformer to isolate the ground of one of the units. It would be cheaper to do this for the preamp rather than the amp.
The trick is to have the audio ground of either the amp or preamp be separate from the chassis ground, with a resistance between the two that allows the audio ground to float at chassis potential, yet is a high enough value to prevent significant ground loop currents. 100 Ohms is plenty high enough! But because the amp is using the chassis as ground you'd have to find all the places the circuit is grounded to the chassis, this kind of repair (since this really should have been done when the amp was built) might be tricky. A technician might be able to look at it and let you know.
Plan B: get an isolation transformer to isolate the ground of one of the units. It would be cheaper to do this for the preamp rather than the amp.