I'm actually kinda lazy to explain this
Most people don't try to fix or build a system around their HD800 anyway. And sometimes when they do fix the tonality, these "fixes" can also eliminate HD800's technicalities (detail, soundstage size).
Here it goes (this is off memory):
- Both have very top quality bass. Yes, HD800 has bass details that surpasses TH900. It also reaches as low as the TH900 (maybe TH900 just slightly lower). It is just so lacking in quantity. TH900, however, has that unique thunderous slam that HD800 just can't have. No other headphone does, to my knowledge. Maybe it's the driver characteristics. Maybe because it is closed-back too.
- Both have perhaps the best quality treble I have ever heard. To me, even better than ZMF Verite, Auteur. Much better than any planar I tried. Only the Utopia can match it. But both have too much quantity (at different FR. HD800 at 6k. TH900 at around 8k, if I remember correctly) It is easier to fix TH900 treble. Schiit Loki did for me.
- Both have recessed mid. HD800 is much more subtle, and no need to fix. TH900 is a problem though. That is why people call it a "Fun" can. I think....something with this kind of quality sound should never be called a "fun" can. It is a formidable and technical can.
- Both are kings of soundstage. HD800 is king of open-backed. TH900 is king of closed-backed. I compared these two comprehensively in the past. TH900 has by far the BEST soundstage in any closed-back headphone. I don't think it will be beaten anytime soon. It is not so much on the size. Yes it is wide and deep, but what amazes me with the layering and imaging. It is even better than the HD800, though HD800 wins in soundstage size.
- Both have a lot of air. TH900 is the airiest closed-back headphone I know. But HD800 is the airiest headphone I know. It still beats TH900 by quite a large margin, due to being open-backed perhaps.
- Both are geared more towards female vocals. Male vocals sound great on them. But female vocals sound even better to me. I prefer male vocals out of Audeze or Abyss.