Further improvements going along the lines you mentioned above. The M50 is not a headphone that benefits much from amping but the ones you mentioned above geometrically increase with the E9. In addition to a few more changes.. E9 is a great amp, makes the mids a lot more liquid and effortless compared to the E7. The congestion disappears slightly, becoming a bit more open, but it's not a drastic change.
I was curious about this. There seems to be a consensus that the e7 is a bit warm. I have a700's which tend to be a bit warm on their own, and I was worried about some bad synergy there. This didn't seem to bother the op though. I was thinking the E9 would solve that problem, any other thoughts about this? Do you think the warmness is mostly from the DAC or the amp in the E7? I'm afraid of spending the extra for the combo from Micca's if all the E9 does is make it louder with any real SQ changes (volume is almost certainly not a concern).
Oh, and speaking of making it louder...would the E9 be too loud for efficient cans or IEMs? If its designed for high-impedence and people are listening to things like hd650's on the low gain setting I'm afraid it'l be too loud, or else I'll have to keep the amp so far out of its "sweet-spot" (which I read somewhere was around 9-11 o'clock) that it wont reach its potential. I tend to listen to things much quieter than many other people I've met (I like having good hearing and I have hp's on nearly all day).