Unsurprisingly, the hf2s sound very much like the ER.4 microPros, which is no bad thing, especially since they're considerably cheaper than the ER.4s too. If you're a fan of heavy, over emphasised bass response, then you're not going to like the hf2s. Like all Etymotic earphones I've listened to, the hf2s are all about clarity and accurate audio reproduction, but many will equate that to a lack of bass and dismiss them, and that would be a serious mistake.
If you want to hear the hf2s in their element, you need to throw some more acoustic material at them. Wild Wood by Paul Weller instantly showed just how good these earphones are. The acoustic guitar and background percussion are rendered beautifully, as is Weller's distinctive voice. Listening to a track like this makes it clear that a bias towards low frequency is nether necessary or desirable.
Changing tack completely I fired up Cochise by Audioslave, a track that recently got a lot of exposure due to the Iron Man movie. Again the hf2s did a great job, reproducing the heavy guitar riffs and Chris Cornell's vocals with an impressive amount of power, but not at the expense of the percussion, which is relatively far back in the mix. However, the slight lack of bass response is evident here, especially when I listened to the same track on my triple driver Shure E500s directly afterwards - but let's not forget that there is a major price differential between the E500s and hf2s.
Next I cued up Future Sex/Love Sounds by Justin Timberlake - when this guy went solo I really wanted to hate his material after he subjected us to ‘N Sync, but unfortunately he ended up producing some pretty damn good stuff. What Goes Around Comes Around is a fairly complex arrangement, but the Etymotics didn't miss a beat, despite the plethora of strings, heavy beats and strong vocals. Again, there's less bass than you'd get from a set of dual or triple driver earphones, but overall you're getting a well balanced sound.