Haha, I'm the opposite, selling the IE8 and keeping the UM3X.
Really, they're both excellent earphones. Neither have any significant faults. It will really come down to personal preference more then anything else.
Some differences:
Frequency Response:
IE8 - slight midbass-lower midrange focus, makes for a warm, slightly bloated/full vocal signature
UM3X - slight lower treble focus, makes for a slightly prominent upper end, slightly bright vocals
Both are relatively flat and extended in response otherwise, basically IE8 low frequency focus, UM3X high frequency focus, both slightly but enough to be noticed.
Sound Stage:
IE8 - big, spacious sound stage, concert hall effect like you're sitting up in the seats watching a performance, big size, minimal sense of depth, layering, spacing
UM3X - intimate, in your face sound, like you're on stage with the performers, outstanding sense of depth and layering, break out the tape measure accuracy.
Pick your preference, up close or far away. Neither are bad, but each orientation carries its own traits.
Dynamics:
IE8 - slightly laid back presentation, good note, good energy, but lacks some edge/bite on the note, not muddy, not slow, just slightly softened
UM3X - high energy, sense of power, better sense of dynamic range and breadth of range, very good in differentiating subtlety and gusto.
Again, pick your preference. The IE8 doesn't falter, but it also isn't effortless in breadth. It has a slight softness to note energy and edge. It does make the IE8 more pleasant, less fatiguing, and with a lower chance to offend. The UM3X offers more sensation, more raw energy, more breadth.
Both are similar in capability otherwise. Neither lack frequency response, both readily realistically producing notes from 30Hz to +15kHz. Both offer a good level of detail, cleanliness, openness. Both offer excellent bass quantity and quality, and both provide a visceral experience. The IE8 offers a little bass emphasis while the UM3X treble emphasis. The IE8 is a little more laid back while the UM3X is more in your face. The IE8 creates a big, spacious space while the UM3X is up close with a great sense of depth.
Just to stir up the mix a little, the Triple.Fi 10 is somewhere in the middle of both, mixing a number of the qualities together. From a cold, calculating standpoint, is is the better IEM versus the IE8 or UM3X. It has a flatter response, both a spacious sound stage with good location and depth cue, outstanding highs, natural mids, and good bass. It doesn't have the dynamic breadth of the UM3X nor quite the bass extension subsequent visceral sense. It is more "correct" then either.
