We cannot conclude anything from the posted plots. Why 2 of the same plots ?
A: We don't know if the dummy load or a resistor was used as a load.
B: mr. Townshend should have made a reference measurement where the input of the ADC should be connected to the output of the amp so we get a real reference in the form of the sweep. The 55.5mV says nothing,
we don't know how sensitive the ADC was set, we don't know if the voltage level is referenced.
C: mr Townshend conveniently forgot to mention the
cable length as well. (this could have given a clue about calibration)
D: About the dummy load: The posted schematic won't have the impedance posted alongside. Amir's sim is better. Why is the plot from mr Townshend incorrect or the schematic is incorrect (has a Boucherot filter installed ?)
E: Why not a comparo between regular speaker cable and the Isolda instead of the single strip of isolda cable which measures FAR worse that any speaker cable.