I agree with what has been said. If you're coming from iBuds, I don't know if you'd notice the difference b/t Mid-Range IEMs and High-End IEMs since you'll already be listening to music in a way you're not used. That being said, I did a fair share of the upgrade path and the E500s are the best universals I've heard to date (still need to try the triple.fi's but I don't like how the design doesn't sit flush). The SF5Ps are phenomenal sounding (yet a good notch below the E500s) if you can get past the design and sticking out of your ear 3 inches. I have not listened to UM2s, so I can't offer any advice there. As far as a Mid-Range IEM, I'd take them into high consideration purely based on comfort and design factors.
Amping IEMs is a touchy subject and in my experience, the improvement really depends on what IEM you get. If you do get the E500s, I'd highly suggest a Hornet M to go with it. I find it difficult to listen to the E500s sans amp, now that I'm used to it. So difficult, that I need to EQ it if I don't put the amp in the chain. On the other hand, I didn't find much improvement amping the SF5Ps and actually preferred to take the amp out of the chain due to portability concerns. One thing that most people can agree on though, is that an amp will/SHOULD give you a completely black background. Plus the fact that you'll need an amp anyways if you want to correctly (no lineout/attenuator BS) make use of the lineout (which will increase your SQ considerably).