Disclaimer: This is Inks' pair!
Etymotic Research has been best known for their extremely strict quality control, and that is why their flagship IEM, ER-4 is pair-matched by their engineers. Although the official tolerance limit is 2 dB, almost all of ER-4's interchannel deviation is usually within ~1dB from 20 Hz to 10 kHz. Such accuracy is absolutely critical, especially when the IEM is used for professional/research purposes.
However, that is also the reason why ER-4 is quite steep in price, keeping regular users unapproachable to Etymotic Research's great quality & fidelity. In order to mediate the situation, in January 2002, the manufacturer re-branded Star Micronic PH-001A, and sold it as ER-6. ER-6 was as accurate as ER-4S, and was very affordable too with an MSRP of $140.
And recently, some of my fellow Twitterian named @Fontek_Research reported that Knowles ED-29689 measured very different to what's installed in ER-4. Can this IEM possibly be an economical version of ER-4?
|Note: measured with a pair of ER38-18 sleeves|
CON: Lacks a good amount of sub-bass, but the distortion figure is definitely improved from ER-4 & ER-6, which makes HF series an Etymotic Research product with the least amount of distortion.
ON SECOND THOUGHT #3: Other factors are identical to those of ER-4, so please refer to ER-4B analysis for further details: 1, 2, and 3