I have the e4s and a porta corda. I like the e4s fine straight out of the iPod, but they are improved with the amp. I would not have cold feet here; get the e4s, then get an amp if you really want added ooomph, and be happy. I particularly agree with comments in posts #13 and #15.
Edit: I A/B'd my shure e4s and e5s today out of the porta corda, and am starting to think I prefer the e4s. The e5s were indeed fuller and richer and had far more bass, but the e4s were cleaner and faster and had more high-end detail. The sounds are quite different, and I think moods and tastes will vary for preferences, but the e4s have a nice, fast, clean sound, with tight bass that is quite good, IMO. I listen to hard alternative rock, by the way (alice in chains, incubus, a perfect circle, nirvana). If you prefer bass, then I think the e5s or from what folks say, the UM2 or the super5.pro would be the way to go. I bet even my own mood and preferences will vary like month to month or so as I switch off between these two. Right now, gun to my head, I'd take the e4 over the e5. Could change though, but that's why I'll likely hold onto both.
- walk