I can share some thoughts based on the TH900 Mk2 in stock form.
They’re an interesting headphone, and certainly very pretty. I went with the Emerald Green version as well - and it’s a bugger to photograph.
Right out of the gate two things stand out, a pronounced “U” shaped or “W” (with a smaller central peak) shaped signature. Bass, sub-bass in particular is significantly emphasized with excellent slam, growl and rumble. Sadly the bass levels are such that they bleed into the mid-range. And the top end is also quite pronounced - a bit too much so in fact. The resolve well, but the mids get lost somewhat so that’s not as apparent as it could otherwise be. They’re a lot of “fun” to listen to … but they can get fatiguing quickly.
vs. the ZMF Eikon - the ZMF headphone is far more neutral - in fact they have the best tonal rendering of any closed-back headphone I’ve heard, with just a hint of warmth. The TH900 Mk2 are somewhat more resolving, have faster transient response and offer more slam and a little better handling of rapid, large, dynamic swings.
vs. the Focal Elear - now you’re comparing two headphones with similar degrees of sonic flaws. They have different flaws. But they’re unmissable in both cases. Both have significantly elevated bass response, but the TH900 Mk2 is more convincing, especially for sub-bass presence, in that realm. The Elear wins in overall sonic agility, and is less peaky overall, with a smoother and less biting treble delivery. But the Elear still has a wonky tonal profile that requires the application of EQ (or Elex pads) to make listenable for me. With something like True-Fi, they’re basically a prettier Elex.
vs. the HiFiMan HEX V2 - the HEX have much better tonality than the Fostex. The Fostex have a much faster rendering, more prominent and impactful bottom end and a more aggressive sound - the HEX sound almost slow in comparison.
I really enjoy my TH900 Mk2 with music that really fits their signature. And they’re undeniably beautiful. But they are flawed. Even the similar but much cheaper TR-X00 Purpleheart are more neutral. They’re a high-energy, exciting, in-your-face listen, which is great when you want that, but otherwise can get fatiguing very quickly. Sticking TH610 pads on the TH900 Mk2 helps a fair bit, but is not a panacea.
(My comments here are based on currently owning, among others, the TH900 Mk2, ZMF Eikon and Focal Elear and having spent lots of time with the HEX v2).