IIRC , it will have been a requirement on importing that receiver that it be fitted with a 3 pin plug for British conditions, including an earth to the chassis.
Over here, the neutral line is returned to earth at the bus rail on your power distribution board. Removing the neutral link at the power board is very dangerous. In the case of a two pin power connector, we would normally assume the chassis of the equipment is double insulated, preventing a path to earth through the user, if the chassis goes live. The instant the chassis goes live, the main fuse or breaker would pop, since it would be a short circuit- supply to earth.
Your problem is too many earths , with the earth potential not being the same for both devices. A hum loop is caused by a circulating current in the earth return BETWEEN the two devices.
You have to lose an earth somewhere..My only recommendation at this point is to get an electrician to meter out the tuner power connections to find how the earth is connected, and whether it is in fact , double insulated.