Pretty simple. You need to run power to the E9 through the included power cable. Make sure USB charge is on on the E7 (default setting and either dock it on the E9 or straight to computer with the included cable for the E7. To properly use the combo you will need to dock the E7 on the e9 as so
and make sure the combo is set as your default sound. Control panel-sound-USB audio DAC or on Macintosh, System preferences-sound-USB audio DAC. Make sure both are on (you have to turn them on seperately, and plug in your cans. Play some music and enjoy!
HINT: make sure you are using a USB-B Cable, that is the one included with the E9