I usually just have my E7 docked on my E9 so the bass boost functionality has been un-abused thus far. Earlier today I decided to use the E7 by itself and wanted to boost some bass on my hip-hop music. And what did I get? Distortion. And I was only on level 1. Levels 2 and 3 sounded absolutely craptacular. BTW, this is from listening on my laptop with the E7 as the DAC. Programs I tried were foobar+WASAPI and PotPlayer+ReClock. Both had the same distortion problems.
There was something definitely wrong because my FiiO E5 w/bass boost sounded very good. E7 should at least be on par.
But I did not give up hope on my E7. The next logical thing to test out was Aux In. So I plugged in my little Sony Walkman MP3 player, tried out the different bass levels on the E7 and, to my surprise, NO distortion! Even on level 3 there wasn't any (well maybe some).
So right now I'm thinking, "That's weird." I don't see why I would only get distortion when using the DAC of the E7. Perhaps there are shielding issues in my unit? My unit is straight up faulty? Can any other FiiO E7 users chime in?