It isn't the Luxembourg effect (which was crossmodulation of Long wave Luxembourg station in the ionosphere onto other stations, so could be heard in the background) . The mix on 729kHz is intermodulation occuring at either the transmitter site hosting the stations concerned OR the receiver. It could be either. Could very easily be happening at a receiver with a broadband aerial amplifier.
See if it can be heard on a good portable radio using a tuneable MW loop alongside, If the mixture gets 'peaked up' in strength by tuning the loop to 729 (NOT 909 or 1089), it does indicate it is 'in the air', rather than happening IN the receiver.