I have had an E7 for several months and have just added an E9 to drive my larger headphones (AKG K401 120ohms).
I tried out the E9 and E7 with three different inputs, usb via the E7, line out from a Bose CD player and headphone out from a Cowon X7.
Firstly usb, I was surprised, and somewhat disappointed to find that the E9 could only just get to an acceptable volume level, even with the gain boost switched on. The E7 on its own was considerably louder!
Secondly E9 aux in from the Bose and the Cowon. Set the volume on the latter to be similar to that from the Bose's line out and played them through the E9. The Bose was very much louder, the Cowon needing both volume settings much more than half way to be acceptable.
Lastly E7 aux in as above. Again the Bose much louder. With the Cowon, the E7 didn't seem to show any gain at all, even at full volume!
I am guessing that the last two may be something to with impedancies, but the first I find completely baffling as the two units were designed to work together.
I would be much obliged if anyone could throw some light onto the above issues.