Here is what I am getting. I included 10 feet of 12 AWG cable (20 ft round trip). Since the AHB2 has a high damping factor at low frequencies, the cable resistance is the main cause of the deviations of FR (the bigger difference is at high frequencies, where the AHB2 DF comes down and the F208 impedance is also lower than with the simulated speaker). So not much difference between mono and stereo except at higher frequencies. The delta is what should be applied as EQ for an ABX test (correct?), but I can't imagine there is any audible difference in the bass, which is the OP claim. I think any bass difference would have to be from something else besides output impedance effects. I am not sure doing ABX with a filter made from this FR delta is worthwhile to do. Recording speaker voltage would be better.
View attachment 136363
edit: remade the chart to see effects better