Mike,
I think you nailed it in your first post when you commented on the source impedance design of the Directors. That is, they are designed to take much, much higher impedance sources.
Unfortunately I cannot find specifications, let alone a schematic for the Directors, but I am guessing you have inadvertently created a gentle high pass filter with the Allen Avionics AGL-600/Director combination. Both devices are are inductive and the source impedance to the Director is massively different to what it is designed for. It makes sense that a inductive/resistive response could change over frequency.
Regards,
Simon.