Okay, some misunderstanding here I can see.
First, L7 extract the analog line-out signal from iPhone, and allow you to amplify it with an amp. This process doesn't in anyway involves the DAC of E7, E07K, or E17 (or and DAC after the L7), as the iPhone's DAC already decode the digital signal to analog sound before sending it to L7.
Second, you want an amp because you want more power, either in voltage, current, or both. iPhone's internal amp is fine, but it is not as power as a dedicated amp. So what if your headphone doesn't actually need that much power? Well, that benefit will be small in the case, if any at all. So does it mean you will never notice any difference? Well, it depends on a few factors, but generally there will be some difference, and even a small improvement. But whether it is worth your while is for your own determination. The rule is, the more demanding your headphone is, the more benefit you will see on an external amp. How do you know whether your headphone is demanding? From the spec, and from your own experience. What if you are a novice and don't know whether your headphone is well driven or not? In that case, you can risk and buy an amp to satisfy your curiosity, or choose to be happy since you didn't find any problem in the first place.
To make the right decision requires having the right knowledge in the first place, and most of what you need to know has already been posted in the forum, so do enough research and you will eventually see a clearer picture.