[Impression] FiiO E7 (warning: lot of pics!!!) - Page 7

Assuming you mean getting the line-out from E7, then the question isn't about whether it would be enough to drive 'something' as it doesn't really drive any thing. Line-out is usually loud because it has the best dynamic range that way, not because it needs to be powerful to 'drive' the next in line (which will be an amp). Line-out signal doesn't usually has much driving power any way since the actual power is provided by the amp. In this case, it will be the power amp inside your z-2300.

A blind guess: E7.

I want to point out that there are two normal ways to upgrade the hardware of a portable rig: First is the easiest and most effective (or efficient) way: upgrade the headphone. Second is upgrade the source, including adding an amp. Assuming you have a sufficiently satisfying headphone you want to keep, you can either upgrade the DAP to a better sounding one, or add an amp. The point of adding an amp to a DAP is based on the assumption that you can get a better sound out of the DAP. (1) To do that, you will most likely need a line-out (which iPod has) and the amp you are going to get must be better than the internal amp of the DAP.

(2) Another way to look at the same issue is, the headphone you have need more power to perform at its best, so adding an amp helps since most DAP doesn't have enough juice.

In situation 1, you get better sound as you have better sounding amp section, but your headphone doesn't really benefit that much from the extra power and control. In situation 2, you get better sound because your headphone is sufficiently powered, though you may or may not using a line-out to feed the amp. For the ideal situation, you will want to have both together - improve the source by having a better amp section (via the added amp) while the headphone also benefit from the extra power and control.

The point is, you may or may not be in the ideal situation all the time, and it is important to do your research to find out whether a headphone or an amp upgrade makes more sense in your rig.

Not sure how well it will compare but I consider E7 to be fairly cold and analytical amp.

Bass boost Lv1 and Lv2 definitely add more warmth to AD700, but Lv3 takes away too much upper freq. to my liking. Bass wise, the quantity isn't too much even on Lv3 as it simply doesn't boost enough to turn the bass light AD700 to something else. Personally, I quite enjoy BB Lv1 with AD700 and think they sound pretty good together.