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).
1. Number of drivers is something to consider, but not necessarily an indication of performance and quality of sound on its own.
2. More crossovers also can mean more problems. Getting a crossover *just right* so it sounds natural is hard to do. Even harder in an iem. The best crossover is not needing one at all (Which is why a single dynamic driver, well implemented, can sound as good or better as 3 or 4 B/A drivers).
6. within reason - *most* people cannot hear 20-20. More important than the range, to me, is the response graph showing how it presents the music. bass heavy? flat response? peaky and tinny? etc. IMO - always aim for a flat (uncolored) response.
7. Anything under 80ish ohms won't be a problem from portable unamped sources (and if you do go with amps, something between 50 and 80 gives you more flexibility without risking impedance mismatches than an uber-low nominal impedance - helps noise floor too). Ultimately though, I wouldn't worry about this one too much either.
Edited by liamstrain - 10/12/12 at 12:04am