Also impressions of the heir 4.ai show them to have a bass response north of neutral. Personally I find coloration to limit the versatility and usefulness of an iem especially when listening to many different genres.
The ER4S is backed by much actual research aimed at producing an iem that matches reference monitors in a sound treated room. Frequency response graphs are easily found, showing them to be the most neutral thing on the market except maybe the fitear tg111 which costs over $1000.
The problem with the ER4S is that it's not easily powered so it requires an amp. In this case it's brother, the ER4P is recommended as it has nearly the accuracy but can be used with portable devices. The discontinued ER-6 (ebay) is also close to the accuracy of the ER4S while working well with portable sources.
The isolation of the etymotics is also the best there is. I never knew what I was missing with my other iems. Wearing them around town and at the gym is really just a much better high fidelity experience imo.
I'm hope this helps. It looks like you are looking for a shortcut to really great sound quality, and this is what I would do if I were starting over knowing what I know now. This is my personal experience with these iems and if one is looking for neutrality like I've been then the etymotics are really the way to go. There is a lot to read about them however, and there are certain variables like fit and cables that should also be considered. Good luck.
Edited by gnarlsagan - 10/9/12 at 6:52pm