Just got some new custom IEMS (my first) and noticed something rather odd. I've had to tune my source device, the Sansa Clip Zip to about 10 points more treble as the default untouched sound is muffled, almost muddy sounding.
I'm wondering if this could be fit related as it is hard or impossible to tell what the tips are doing inside the ear, although they certainly feel comfortable (silicon, which can be more mushy as well).
When I put the treble up they seem to sound more 'normal' as in the vocals are clear as are some of the lighter toned instruments. The weird thing is I've never had to tune the sound for any IEMs before which prompted me to ask here.