The gain on the E7 is about 4 dB at maximum volume (60). 50 is where you get unity gain (0 dB), Which means that the signal entering the E7 has the same voltage as the signal at the headphone output. Some high-impedance cans like the HD600 and HD650 can sound good on a portable player if the track is loud. Unfortunately, there are are plenty of quiet tracks with wide dynamic range; those won't get loud enough on an HD600 even when an iPod is at maximum volume. Adding the E7 buys you only 4 dB more signal, so it's probably not that useful if you want to bring 250+ ohm cans with you.
The E7 can handle low-impedance IEMs with no problem. It plays them with low noise and distortion. I can, however, hear some hiss on my most sensitive IEMs when nothing is playing. When I listen to binaural recordings using the E7, I still hear precise imaging. But that's more a function of the headphones than the amp, as long as the amp does no harm. The E7 will deliver a flat fequency response to almost any kind of load, even balanced armature IEMs. The E9 can't do that. If you use the E7 instead of your Cowon players' built-in headphone amplifiers, you might hear slightly more output in the lowest two octaves.
I looked at your headphone inventory and I don't see any high-impedance full-size cans. So you don't need an E9. You do have several IEMs, but most of them have dynamic drivers. An E9 should play those just fine. But its response won't be uniform with the ones with BA drivers. On single-driver IEMs like the HF5, you might hear more treble, especially if you plug it into the 1/8" jack. That won't happen with the E7. Or the E5, which you already have.
If I were you, I'd hold off on buying anything right now. Your portable players and your E5 can drive your IEMs just fine. Sure, the E7 seems to be a well-engineered product. Its problem is that it increases a portable player's volume by only 4 dB--probably not enough for 250+ ohm cans. Also, iPads and the newest iPhones now have pretty good performance when driving low-impedance IEMs at typical listening levels. For me, bringing a portable amp isn't worth the trouble. Your priorities might be different. So where would the E7 be useful? To me, it makes most sense as an external USB audio interface for a notebook computer with really bad built-in audio. But even those components are slowly improving.
Anyway, I noticed that you've amassed a lot of gear in a short time span. I hope you're giving all of them enough time to be enjoyed. If you need more reassurance, I've compared the E7 to my iPad. I don't hear much of a difference on all sorts of cans. If anything, the E7 has slightly more hiss.