Just looked at specs on ATH-AD900X ... impedance is relatively low (as you say, 38 ohms), plus it has a relatively high sensitivity of 100 dB/mW. The drivers are pretty large (53 mm) so more material to move. But still, E10K is probably plenty good for this.
I always found the E10K to have above average bass--way more than you would expect just looking at the little thing. I was never tempted to try the bass enhancement switch...no need.
Re mids and highs, when I got better headphone amps (starting with the Lake People G109A), I realized that what I had been hearing was not truncated/missing frequencies, so much as subtle timbre cues & detail within those frequencies. Bigger amps w/bigger power supplied tended to give not just more detail, but also fill in each instrument's sound, compared to the E10K.
That includes the bass. While the E10K has good extension and impact, it's harder to hear those subtle harmonic/timbral differences between instruments on it (ie, electric string bass sounding very different from acoustic string bass vs synth vs marimba bass, etc).
I've never been into treble, you know, the kind of "air" that goes up to 50 kHz. I suppose the E10K would fail to go as high, or with the subtlety, of bigger/badder amps known to "do treble" with great finesse.
I never played around w/the 3.5mm line out, to check whether the "sound" of the E10K was more due to DAC or HP amp.
Still, we're talking about a ~$75 DAC/amp smaller than a pack of playing cards. It helps to keep a sense of perspective here...