OK, so I just spent some time with the TH900 and the Elear, both plugged on my WA7tp with upgraded tubes (I found that they both sound better on this amp).
I mainly listen to EDM, a genre where both headphones are supposed to excel, but for good measure I also played some rock, jazz and blues.
The TH-900 has all of its usual qualities: a vast soundstage, amazing transparency in the mids, great separation, incredible bass. But its defects become apparent when I switch to the Elear: the sound is suddenly much less dry, the highs less piercing, the listening becomes instantly easier, more relaxed, but without losing any of its detail. The TH-900 is a lively headphone but after switching to the Elear its aggressiveness becomes apparent. The Focal has a notably darker sound, but for me in a good way: mids are fuller, instruments sound more natural. The music just seems more composed and mature. I miss the pure energy of the bass of the TH-900, but it's traded for a more englobing bass, that makes the Fostex's one seem more like a lateral punch. However the rumble of the TH-900's bass is still untouchable.
It's easier to follow individual instruments with the Elear, the sound is so dynamic its makes some of them (piano, guitar) seem palpable. On the Fostex it's more analytical, still very detailed but less musical. The Elear really has a kind of analogue sound that makes it very natural and pleasing to the ear.
All in all, switching back and forth I started to get the impression that the Elear sounds like a grown, mature man that keeps composure in every situation, when the TH-900 seems more like a technically adept but shouty kid. But with a mean punch!