Good sound stage and separation...
-RE0: nope
-PFE: nope
-TF10: yep
-ER4: big yep
-Denon C751: yep (you should look at this as a budget choice, low $100 range, very good sound, sort of a poor man's IE8.
The hard part will be buying an earphone that actually produces a good sound stage. Most headphones out there are pretty mediocre when it comes to this.
Frankly for the going rate of the TF10, I'd buy the TF10. It's a better product then anything else you will be looking at in the price range, anything.
On the cheap, I would pick the Denon C751. It has a very life-like sound, excellent sound stage presentation, and offers a well extended response. Price hovers just over $100 typically and is a great deal. The only major downside for them as an IEM is that they don't isolate at all, go figure. They also benefit from a slight tip change as the frequency response balances out well for the slightly midbass and low treble peaky sound using the stock, tiny single flange tips. They're just much better balanced when not inserted so deeply.
For the $100 price range, I'd lean you towards the Denon. For the $200 price range, I'd lean you towards the Triple.Fi. For somewhere around there, the ER4S or P would be very nice options, although, I don't know if the ER4 is really the sound you're shooting for. It is the defacto reference IEM, and I would suggest to anyone getting into head-fi to at least try a pair for a while. For a 15 year old single BA driver earphone, it is still one of the leaders in terms of sound quality, balance, and stage presence. If you're worried about bass, you might opt for the Westone UM2, I think around $200, but it uses the same driver as the ER4P for the highs but with an added bass driver to up the low end. It's apparently pretty darn good. I've used the UM3X but not the UM2. The UM3X is one of my favorite earphones but it's a bit outside of your price point. You might be able to find a UM2 cheap enough though if you hit up the for sale section.