You can do an experiment yourself, and EQ down 3-4kHz by 6 dB, while boosting 800-2k by 6 dB (a 12 dB swing should make things pretty obvious) — you should very clearly hear that vocals are some how “brought forward” and that they sound “full”, but because harmonics and secondary formants are recessed relative to their fundamentals and primary formants, these vocals will sound “dulled” and slightly too “warm”. This psychoacoustic effect should work on any in-ear that doesn’t have significant midrange coloration by itself.
The Primo 8's sound is neutral and clear -- there's no boosted bass or exaggerated treble; it just sounds right. The clarity is unforced, and that's a rare commodity nowadays. Take the Cardas EM8513 in-ears ($425) -- they have more bass and overall detail, but switching to the Primo 8, the sound is far more natural. It's also more comfortable to wear for extended periods of time. The Sennheiser IE-800 in-ear headphones are more transparent and open sounding than the Primo 8 or EM8513, but the IE-800 sells for $1,000! Again, the IE-800 succeeds by being even-tempered in its sound; nothing jumps out or annoys -- the balance is spot on. Even so, the Primo 8 is more comfortable, and until you compare it with headphones that sell for double the price, the sound is very respectable. The Primo 8 is a true audiophile headphone, and while it's expensive, you'd have to spend a lot more to get something better.