Regarding the E9 compared to other amps, I currently have the E9 and the MP5 with me right now. The Mp5 was a $500 amp a few years ago. What I have is the revision with better internal parts and the American version tube sockets. I will say that the E9 holds against the MP5 very well. In fact I like the E9 better than the MP5 in many respect sonically.
Here's a quick write up I made some time back. BTW Mios = K701
M11x -> Winamp -> E7 DAC -> E7 Amp -> Mios
M11x -> Winamp -> E7 DAC -> E9 Amp -> Mios
M11x -> Winamp -> E7 DAC -> MP5 Amp -> Mios
This chain let me segregate the E9 amp so that I can see how it compares to the other amps I have on hand. With the E9's wealth of inputs and outputs, doing this was easy. As a bonus I also tried the HeadAmp4 which is a favorite for splitting the audio signal to 8 different headphones.
In general, the E9's amp section has a pretty neutral sound, but there is added body all around. The bass is tight and full, and doesn't sound anemic or boomy. The highs are sparkly and clear, and there is quite a bit of air in it as the highs do extend pretty well, not as well as the MP5 but but there isn't much roll off to my ears.
The thing about the E7 is that it really can't drive the Mios properly. It gets loud enough, but the sound is kind of hard and crackly, and the bass is there but a messy muddy clump. The highs are a bit shrill and remind me of rasping glass. However out of the E9 the all of these problems disappear. Instead you get a very liquid mid-range which really brings out the Mio's strengths, as she is a very mid-focused can.
Compared to the MP5, though, it is a very different story. Both the MP5 and the E9 have mid-focused sound, but the E9 is a more up-front kind of sound, compared to the MP5 which has a crystal and very airy character. The E9 sounds ever so slightly wetter than the MP5, and the MP5 overall has a slightly harder sound to it where as the E9 feels a bit smoother. It is this wet quality with a good amount of decay I believe that gives the E9 its liquid quality. Sounds just seem to flow one into another from its headphone out.
I've been testing the E9 with more cans since then and I'm finding I prefer the E9 more and more to the MP5. Sure, the MP5 these days only costs about $200 compared to how much it was before, but the E9 is still like a $100 amp. It's amazing how well it does. I actually went and ordered some new tubes for my MP5 because I was no longer happy with how it sounded. I may end up selling it soon if I can't roll it into a sound that betters the E9. Funny though, the tubes I ordered cost more than the E9 itself.