The mod is an inverse filter that compensates for an individual's hearing loss. It has to be fit like a pair of eyeglasses.

    Perhaps you could pick a few photos that you like and I could explain what is going on.

At this link below are photos detailing circuit modifications to a FiiO E12DIY to help a hearing impaired person hear their TV.   I do not have sufficient privileges to upload photos to Head-Fi.  Perhaps someone else can select a few and upload them for me.   I was hoping that these photos...