I've never used the E11, but most claim that it's a cleaner/stronger amp than the E7 and it also adds the bass boost function that I feel the Ad900 needs, only slightly though. I have the E7 which I love, which has it's own advantages over the E11. I know that the E7 will definitely power the Ad900 to their potential for sure and you have the added bonus of using it as a USB DAC/Amp. I hope that doesn't sound confusing, let me try to explain better:
E11
Advantages
-Better amp for potential future upgrades, especially for portable use. It's said to be able to power some really power hungry headphones off of battery alone.
-Removable/interchangeable battery
Disadvantages
-It's rather one dimensional since it can only be used as an amp.
E7
Advantages
-Acts as a decent portable amp
-Great battery life
-Acts as a USB DAC/Amp when plugged through USB.
-Can dock into the E9 to be used only as a DAC while taking advantage of the great amp in the E9.
Disadvantages
-Not very powerful as a portable amp. The E11 is much stronger.
God that probably came out a mess.