From your measurements the IEMS seem fine compared to the M50. Perhaps there is a fit issue? Are the totally secured in your ears and appear to seal?
Second, what source are you feeding the amp with? Does it have any EQ settings turned on?
From your description:
I'm also no expert, but it seems there are much more noticeable harmonics on the T1 300MHz and 400MHz measurements.
So it puzzles me, these highly expensive IEMs that received pretty good reviews...