As I understand it, the audiologist prescription includes an EQ map as well as few different types of filters for different situations: music, conversation in crowded room, 1:1 conversation, etc. Hearing impairments are not totally amplitude dips but also comprehension ones. SO I suspect aids have a few extra DSP filters available to the programmer that IEM's don't, while IEM's probably have wider range and better reproduction for the musician's taste.