Just a thought::
IF the volume of the radio station doesn't change with changes in volume control level, that would tell me it is coupling in AFTER the preamp gain stage, which would presumably change the level of the RFI, as well. The fact that it is coupled for ALL inputs helps that conclusion.
Perhaps rearranging speaker wires would help. NO coils, for example.
Do you know where the radio station's transmitter is located? How far? How much power?
If what turns out to be the antenna is at right angles to the transmitter, that will be the orientation of maximum sensitivity to the offending signal.
IF the volume of the radio station doesn't change with changes in volume control level, that would tell me it is coupling in AFTER the preamp gain stage, which would presumably change the level of the RFI, as well. The fact that it is coupled for ALL inputs helps that conclusion.
Perhaps rearranging speaker wires would help. NO coils, for example.
Do you know where the radio station's transmitter is located? How far? How much power?
If what turns out to be the antenna is at right angles to the transmitter, that will be the orientation of maximum sensitivity to the offending signal.