What E3 does is adding a lot of bass and warm to the music, but also take away quite a bit of treble and detail. It fair well on analytical headphone cause it makes it more musical, especially if you are not an analytical listener. If you are, E3 isn't going to be of any help. If you already have bass heavy headphone that are not very analytical, E3 will do more harm than help.

E5 is better in the sense that you get to choose whether you want the bass boost on or not, and of course it also has a much better internal than E3 so it is an overall improvement at such a small price. The down side is of course it is a warm amp even when the bass boost is off. The ideal situation of amping is 'wire with gain' - that is, you want more power and control with as little coloration as possible from the amp. I often find people thinking coloration of portable amp as an improvement. Personally, I would think it is more of a preference rather than an improvement, like people preferring EQ to add more spice to the music but it doesn't actually add more quality to the music nor adding any more power or control.

E1 is what I consider to be the better of the three. It does what an amp is suppose to do, add more power and control without adding much of its own character. That is what people refer as being 'transparent' - it is there to do its job but has minimum impact to the sound signature. I would think if you can't feel any change from adding E1 to an iPod, it is either 1) your headphone doesn't benefit much from the extra power and control, 2) your headphone isn't good enough to reveal the difference, 3) you are unable to detect the change even it is there, 4) you don't think the change is that significant, or 5) combination of any of the first 4 factors.

You should know that, amping is not some magic trick that can turn water to wine. More than often it is a refine of sound rather than an significant improvement. The very real fact is, a $1000 portable might only be 3% to 5% better than a $500 amp. In the audio world, price usually doesn't linear to performance increment. The benefit of an amp is often not in-plain-sight unless you have the time to discover it. It is often easier for those who has the experience to notice the difference which can very well be nothing to another person, as it takes time to train the ears to pick up micro detail. Even for an experienced person, what constitutes as 'significant' is still very much a personal judgment. At the end, it can be that you don't need an amp at all, or perhaps what you want is a much better amp. It is an process of trial and error to find what suits your taste and the synergy of your rig.