Depending on what one wants, a closed headphone upgrade from the TH900 may be:
- a more linear headphone, with better mids - this one should be easy
- one with more defined/better quality/more transparent bass as opposed to bass quantity (it's still a difficult call)
- better treble - this should be easy.
However, as a total package, I haven't yet found an upgrade from the TH900 with a closed headphone. For stock units, I think the Sony Z1R or Audeze XC might be a good upgrade. When modded, I prefer the TH900 over both.
A Denon 7200 may be an upgrade for its linearity, but IMHO needs mods for catching up with the TH900 in the bass and sound stage areas and it's still not perfect either. I guess many people would still choose my modded TH900 over my modded 7200, and vice versa - I cannot decide either.
ZMF headphones might also be upgrades, I haven't heard any of them, but based on reviews and private discussions I am not sure I want to check out that path at the moment; maybe in the future.
The Oppo PM-3 (with bigger pads) might be an alternative with no particular issues, but not really an upgrade.
The TH-X00 variants, TH610 etc didn't prove to be upgrades. I usually don't like Hifiman and Audeze sound, but you might explore them.
If you can include open headphones, these are IMHO better than the TH900 (not a complete list and not in order of quality): Stax L700, 007 Mk1, 009; Abyss; LCD-4; Focal Utopia, HD800S.