I hope I didn't overdo it, but I get a kick out of figuring things out and helping. I don't ALWAYS know the best answer, but usually I can point the way.
I actually paraphrased the headphone impedance info from that certain guy's blog. He does show the E9 in a few of the graphs, and shows the difference in how much amp output ohms has on two different headphones (with less effect on the Sennheiser), and says that if a headphone had a linear impedance response (he didn't know of one) it wouldn't be affected much by the amp output ohms.
I didn't know the E17 kept using battery power even while charging. I was personally very excited about the E11 having changeable batteries and tried petitioning for the E12 to have that feature when it was being designed, but I lost out to the majority that said they'd like to use the amp while charging. But, I came to the same conclusion as you: probably will buy something else once the battery wears out, might as well get my money's worth in the meantime. What I know about lithium ion batteries from training at work, they do wear down over time like a car tire, but not using them at least once a month for a full cycle can also be harmful to the maximum battery health (so exercise the cells for a full cycle at least once a month), and that it doesn't suffer from memory effects like old rechargeable battery types so normally you can "top off" the battery without hurting it.
The O2's design tries to be somewhere between a desktop and portable, and ergonomically isn't successful at either, IMO. Bigger than pocket size, awkward shape, while also having a smaller knob and cluttered port design, and not being as good as a desktop amp could be (though very nice for a portable amp, tho I don't count a transportable amp as a portable... my iMac is transportable too, and I don't call it a portable!).
I don't think I'd ever buy the AudioGD personally, just from the price, Mad's slight improvement in performance, and the issues he had with it.
if the AD700 changes at all from the 10 ohm output impedance of the E09k's amp, it would be a bit of bass warmth that might actually sound nicer