Ah yes, so the factor to consider are as follows:
1. Number of Drivers
2. Crossover (more crossover means better sound quality)
3. Bores (Yes means better sounding)
6. Frequency response (at least within 20-20k Hz range)
7. Impedance (lower impedance means not needing amplification to produce sound clarity).
Please let e know if I need correction in those factors above.
Here's my take on those 7 points:
1. As has been said before, high number of drivers does not mean better sound. Some single-driver dynamic IEMs sound better than 3-way BA. More BA drivers have the potential of sounding better (wider frequency response, less distortion, higher output level), because two BAs cannot cover 20 Hz - 20 kHz.
2. No crossover is the best crossover. :)
3. Bores as in vents? More vents don't guarantee better sound but maybe a more balanced/weaker bass response.
4. Comfort is important.
5. Isolation too, if you need it, but isolation will suffer with vents and also if the IEMs don't fit right (comfort?).
6. FR is very important. Doesn't have to be completely flat but there shouldn't be big peaks and dips. Also, bass might be boosted and treble rolled-off by a few dB (depends on your preference).
7. Impedance and sensitivity is a bit more complicated. Generally you do want a device/amp with a very low noise level if you have very sensitive IEMs, else you'll probably hear a constant hissing noise.
Best would be an IEM with very high impedance and also very high sensitivity, so that the amp doesn't have to provide much current (-> lower distortion, channel crosstalk, battery life ...).
Edited by xnor - 10/12/12 at 7:36am