The E7 is inadequate for your headphones. For one thing, they're rated for 16-300 ohm headphones, not 600. They have a sensibility of 96 dB/mW, which means they need 25 mW to reach a sound pressure level of 110 dB (i.e. LOUD). According to FiiO's specs, the E7 can only output 16 mW into a 300 ohm load, let alone a 600 ohm load!
The FiiO E9, on the other hand, can output 80 mW into a 600 ohm load, more than enough for your headphones! The good news is that you can keep your E7 and use it as a DAC by pairing it with the E9. Make sure to plug your headphones into the quarter inch jack though.
Edited by skamp - 4/1/12 at 2:47am