Based on https://diyaudioheaven.wordpress.com/headphones/measurements/brands-s-se/hd58x-jubilee-massdrop/ schematic. The filter is supposed to decrease the treble spike at 5khz. It is supposed to make the treble sound "smoother" and "fresher" based on what diyaudioheaven says. (Never tried it myself). All credit goes to the guys behind diyaudioheaven for thinking up the filter, I just made it into a standalone pcb. They also make the kameleon amp and modules which can alter the FR even more. On another note, it would be possible
KiCad 5 or above is needed to open this project. KiCad 4 or below won't work.