O_o he is measuring the transfer function of his own ear to what he perceives as flat by using his calibrated speakers and measuring the FR from his ear(which will not be flat due to changes in FR due to the ear's own acoustics). By taking that HRTF, one can put on graph and correlate how the headphone would sound. As for fidelity, pursuing the ideal FR(in this case, the OP's flat response is ideal to his own ear) is paramount to fidelity. If you equalised your headphones properly, soundstaging with them can be improved. In my case, my TF10s after equalisation to approximate the ideal response sounded more open.