You note I gave 2 examples that actually benefit from high output impedance & the manufacturer actually provides the resistor adapter cord with the 75ohm resistance to improve its performane to a higher level. The ER4P & the cord turns it into the higher performance ER4S. Shures best single driver model actually benefits from higher impedance in actual tests as well. Will not work with multi driver ones though due to the crossover. The ER4S is one of the most accurate IEMs available & is more accurate than most full size headphones.
If you have a muli driver IEM you should find out the output impedance of the amp they made to drive it as if yours is any different it will alter the crossover frequency either creating a hole in response if output impedance is too low or a peak if the output impedance is higher than designed for.
I understand more about this topic than you give me credit for. I don't fall for a manufacturers blanket statements to apply to all which they don't. Not saying it don't apply in the cases they test but it is certainly not applicable to all situations. I also question thier testing methodology as they appear to be readings directly from electrical input not measured acoustically so we don't know if it actually improves performance acoustically. A super high damping factor will sink any electrical distortion but that does not always equate to higher acoustical performance. It does not appear that they did any acoustcal tests to confirm better performance in actual sound.
Some speaker types work better with low output impedance (acoustic suspension) & some at higher output impedance (some horns & some ported designs though not all).
Edited by germanium - 1/10/12 at 10:22pm 
