The amplifier in the E7 does a great job of improving the output of just about any player you care to plug into it. We fed an iPod touch through the E7 and were immediately impressed with how much fuller the output sounded and how much larger the soundstage appeared. Many tracks were given an extra touch of nuance, with quieter, subtle background detail brought forward with improved presence.
Even better results were had using a line-level output from the iPod's dock connector, so that only the amp in the E7 was used, which removed a faint hiss previously present and cleaned up the high end a little. Bypassing our iPod's amplifier also left the audio feeling much more natural in its presentation.
The E7 - EQ left untouched at least - doesn't give quite the same warmth as the E5 headphone amp, but it definitely delivers more richness of tone and adds a depth to the soundstage that the E5 can't manage. And the EQ setting works well to improve the warmth of the E7's output should you want to do so.
It's trite and it's cliché to say it, but you really won't want to go back to using the vanilla output of your media player once you've used the FiiO E7 for any length of time. With the caveat that you need to have good enough quality headphones to bring out the improvements provided, it really is a brilliant upgrade in sound quality.
There shouldn't be any worries about the quality of the DAC in the FiiO E7. The Wolfson WM8740 chip is the same as found in the iBasso D2 Boa headphone DAC/amp costing some £100 more than the FiiO E7, and it works just as well in the E7. Listening to a number of tracks first through an iPod and then through the E7 hooked up to our PC the difference is profound.