Oooooh ooohhh *raises hand* *cough* Yay, my favorite rant topic...
There's only one IEM that I know of that gives good soundstaging -
this. It's the Stax SR-001 Mk II, and I really love these things.
Most IEM's are really weak in terms of instrument separation and imaging. I've read that custom molded IEM's improve this somewhat (have yet to hear one), but as far as ER-4/E5c go, if you're looking for good soundstage, forget it. That's not to say that they don't give lots of detail or impact, as the ER-4 is hyperdetailed almost to a fault, and the E5c gives you enough impact to knock your socks off (properly amped and driven).
The Stax SR-001 Mk II is a bit different, though. First, it's open, and isn't technically 100% a canalphone, even though you do wear it like one. Because it's open, it won't give you any isolation, and it will leak a tiny bit of sound (not enough to bother people unless you play them loud in a quiet environment). Their soundstaging is what sets them apart most - since they're electrostatic and not quite a canalphone, they give excellent imaging and instrument separation. At the same time though, they sound full and lush, with a warm tonal balance not quite unlike the E5c. They have strong bass, beautiful mids, and a recessed treble, though not as recessed as the E5c. They have as much detail as the ER-4 overall (less in the treble but more in the mids and bass), however, they have a more musical sound than the ER-4, with a more relaxed way of presenting micro-detail.
Sonically, I'd rate them well above the ER-4 and E5c. I've heard that some people do prefer these to Sensaphonics, but I wouldn't go nearly that far, since the 001
does have a few holes in it's sound (recessed treble and coloration).
In terms of practicality, they're not that good. You need to carry the amp around, and they're not very comfortable, although you do get used to them after some time. If you've got a small mp3 player like a Rio Karma then you can easily fit it and the amp in your pocket - provided your interconnect is flexible enough - but if you've got a bulky, older PCDP, then you won't be able to fit the setup in your pocket for sure. Also, they sound so good with an AC adapter that you won't want to hear them battery powered afterwards
If you want canalphones with a soundstage, this is your only alternative. Also, in IEM's it's the best sound quality you will get for the money. But, you'll have to live with some practicality issues. I love these things and I won't switch to another IEM, but I use them as more of a transportable system than something that I can walk around with (although, once again, if your source is small then this won't be a problem).