truly flat is a VERY relative idea. we don't know what source impedance Shure intended when designing this IEM, then which filter is the right one? and in the end, what does it matter if to you it doesn't sound right? ^_^
to me, at least so long as we're discussing BA drivers, acoustic filters, EQ or added impedance, they're 3 tools to reach the same goal, a frequency response we enjoy. filters are very limited as one filter = one response and you don't have many filters.
extra impedance can be tailored with different DAPs, but also with extra resistors in series, sadly the shape of the resulting EQ will still be determined by the IEM itself so that's a big limitation too.
as for a digital EQ, they will be as good and versatile as the implemented EQ will be in the DAP. sometimes it's pretty good, sometimes it's very limited or simply sounds like crap.
for more fun we can think of mixing all those options for even more variations. in the end the real issue is to have a clear idea of the signature we desire. which can be an adventure in itself.
all we really try to do with some objective approach is to limit the risks of stuff going bad, and making some sort of preselection of the things to try. but as a true neutral signature for IEMs is tied to the user himself, we end up at the same place. what we like, or what feels neutral to us.