The 75 ohm adapter helps the 900s quite a bit. I'm running straight (less variables, and I also like the remote and mic functionality :p). The 4.Ai does improve with the 75 ohm adapter as well (controls bass a bit, helps the upper mids a little more). It doesn't solve the problems though, but no denying it helps them (IMO).
I was looking at Rin's impedance and FR graphs. I don't like it in general but I have to admit the when comparing the 2 graphs, a bit of loading should help almost everywhere.
Swim, you can't use that graph as evidence and them ignore one of mine from a different IEM that you called bass shy.