It took me a while to identify the set of eartips that worked best for me (I ultimately chose the large standard tips), but once I got fit properly dialed in I was treated to one of the most delightfully neutral, full-range sounds I’ve ever heard from any in-ear headphone. While it’s still a little early on in my listening routine to make sweeping pronouncements, I think that in terms of overall smoothness, neutrality, and extension of frequency response, the IE 8 will be able to stand shoulder-to-shoulder with two of the best top-tier in-ear headphones I’ve heard thus far—the Shure SE 530 and the Monster Cable Turbine Pro Copper Edition.
Note, though, that my favorable comments are based on using the IE 8’s in a fairly quiet environment with their frequency response adjustment screws set in the full-neutral position (i.e., with no bass enhancement added). Under those conditions I found the IE 8’s bass response was so taut, powerful, and deeply extended that I could not imagine wanting to add more. That said, however, I would observe that with this ‘phone getting a truly airtight seal is absolutely critical to enjoying optimal bass response and that getting a really good eartip seal with the IE 8s was not easy for me (actually, all in-ear phones are sensitive to fit, but the IE 8’s seemingly more so than most).

In the broader scheme of things, I suspect that one reason why Sennheiser makes provisions for dialing in even more bass is to account for he fact the IE 8’s may be used in environments where there could be lots of loud, low-frequency noise present. In such environments, it could potentially be helpful to turn up the low-end response to “cut through” masking noises in the background.
I was also struck by two other characteristics of the IE 8 that I look forward to exploring in more depth. On one hand, they are extremely detailed and revealing, yet on the other hand they possess an uncannily smooth and relaxed quality that makes them easy to listen to. Frankly, most other ‘phones that reveal as much detail as the IE 8’s do tend also to expose rough edges in the music in sometimes unpleasant ways. In contrast, though, the IE 8’s skew the balance more toward the “gain” than the “pain” side of the equation, which is fine with me so long as critically important sonic details don’t go missing in the process. Finding out just what is gained (and what may be lost) with the IE 8’s should, in sonic terms, prove an interesting and rewarding task.
Next month, I plan to offer a full-length review of the Sennheiser IE 8 in Playback. Until then, let me wish you enjoyable (and sometimes eye-opening) listening experiences of your own.