@Leo888
I did have microphonics with the standard tips, and it's still present, but has settled down a bit with the foam tips - I also have the chin slider just above my adams apple so that helps.
@yugas
I believe with dynamic drivers (like on the FXT90's) the seal you create when you insert the IEM nozzle/tip into your ear causes pressure inside the IEM - If that pressure can't escape quick enough or out of the IEM, the driver flexes inwards (unnatural) and this causes a horrendous crunchy sound (like the sound of cellophane wrapping being screwed up) which can in turn cause the sound to be muffled - The drivers are very very fragile so not much pressure is needed to cause it (if you've ever touched one) - I'm not sure how you can prevent it or why some sets do it and why others don't.
I assume it's down to the individual and what seal they get with certain tips (my left ear flex is WAY worse than my right) -
This is my take on it anyway - I'm no technical specialist