To me the TH-900 & HD800 are complete opposite headphones. They are my end-game pair (for now, lol) the hd800 is very analytical and excels is imaging & soundstage. They have peaked trebles, but their mid & treble clarity are unmatched. Perfect for instrumental, vocals, classical, jazz, & orchestra. The only thing wrong with them is they can sound flat with some music genres. The TH-900 excels in dynamics, with impactful low frequencies when you want to feel the music, while still having that high-end mid & treble accuracy and clarity. It really gives some music genres so much more life & soul. Great for rock, pop, DubStep, and hip hop. A smaller soundstage than the massive one on the hd800, but large enough for 3d imaging and instrument spatial separation.
Hd800 also leaks a lot on sound both in and out. When it's noisy around or late at night to keep sound in, the closed TH-900 takes precedence.
Both great for which mood I am in and music genre I feel like listening to. Both masters at their own elements.
I sold my Beyerdynamic T1. I would say its sound signature is in between the TH-900 & hd800. Amazing with all music genres. Just didn't have that special magic that my two current headphones have.