I have some great news to report for those who may be looking to use the E7 with an iPad: The E7 works as a USB DAC for the iPad!
The FiiO E7 works as a USB DAC, so the iPad volume control no longer functions. As shown in the picture above, the volume control is all the way to the left, yet the volume out of the E7 is unchanged. Here we are driving a set of DT990/600 phones that have a 600Ohm impedance.
The E7 is particularly well suited for use with the iPad because it's USB charging feature can be turned off. With it turned on, the iPad reports that the attached accessory is attempting to draw too much power and turns the interface off. But once USB charging is turned off, the E7 strictly runs from its internal battery and iPad is very happy to output music over it.
And yes, it sounds great. :)