Well, FWIW, I don't think "the balance of X headphone" is the same as saying "X headphone is balanced." In the first case it's basically a synonym for how the headphone is tuned.
As for the second statement, I don't think I'd personally equate balance with linearity, so I think I'd actually agree that the TH900 is more balanced than the HD800 in stock form. It's not more linear, no, but the entirety of the sonic spectrum is presented more even-handedly (a U-shape is more balanced than a "weaponized face tweeter" to my thinking...).
And regarding the U-shape signature on the TH900, it's actually not as apparent as on the D7000 for instance. The bass emphasis is slight to my ears, and the treble emphasis is even slighter. The TH900 is perfectly balanced for me when it comes to residing on the medium between too little and too much. Goldielocks-esque I suppose...