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The problem is, that your description of the setup is still unclear. And I'm only willing to ignore possible return currents through the cable shield, if you tell, that it's unconnected at the other cable end. Is it so? If the cable shield however forms a closed loop, it will affect impedances.You would have to trust me that this is the way the system is hooked up.
If the amplifier is floating and not connected to ground, direct or indirect, will your coax cables external shields interconnection cause a good signal path for signals between the both inner conductors.
The closed loop will almost cancel the conductor inductance contributed by the loop marked below. If you say, you are compensating the loop inductance of each setup, if won't matter at first order. But because the inductance has also skin effect losses, the exact behaviour is more complex.
Another effect of the metal housing is connecting the shield capacitance of the coax cable, which is floating in case of a plastic housing. Although you say the amplifier is "floating", I would expect a finite common mode impedance though. So the shield ground connection will most likely matter somehow.
Your observation, that the error is increasing with higher load resistance would also suggest a apacitance effect. Depending on how you measure the load impedance, the effect of the capcitance may be different. For an detailed analysis, the complete equivalent circuit of the generator output measurement would be required. If we estimate the cable capacitance effect for a simple RC parallel cicrcuit, we get 20° phase shift with 200 ohm and 600 pF, coresponding to 6 m of coax cable.
"the input and output terminals" if they both actually are output in a balanced TX output, then I can understand that it at least to a certain degree is a closed loop. Coupling effects due to half floating shields and half floating enclosure seems a bit undefined and due to this is it likely that high resistive values as load on a balanced output increase your vector impedance uncertainty.
It would open the loop.So what if I disconnect the coax cable shield from the resistor-box case? Will that help?
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