More on this later. I suggested Ralph run a simple test to show whether this mixing product was occurring in his receiver or somewhere else. I asked him to insert the input attenuator on the Drake R8 and let me know how much the interference dropped on his S-meter. He did that and reported the 20dB attenuator caused a 3 S-unit drop in level on the 60 meter band mixing product. This test combined with the fact that the ICE filter had no effect allowed me to deduce that the mixing was not occurring in his receiver.
Mixers are non-linear devices.
If you reduce the power level of the strongest component going into a mixer, the power level out of the mixer will drop by more than the original reduction. That's close enough to draw the conclusion that things downstream of the attenuator are operating in a linear manner. This is confirmed by the fact that greatly reducing the level of the local broadcaster with the ICE filter had no measurable effect on the level of the mixing product. The conclusion is that the mixing is occurring before the receiver and is being re-radiated and received on the antenna. Mixers are essentially diodes.