It’s the huge peak around 6kHz on the TH900 that probably gets most people. The ADX5000 doesn't have that as bad specifically at 6kHz, but it also has some other annoying peaks around 4-5kHz, so it kinda depends on where your hearing is most sensitive which is gonna sound brighter.
I think the TH610 is a good answer, that 6kHz peak is tamed some, but the better answer is probably the E-MU Teak, which tames that 6kHz peak even better than the TH610 while still giving you that Foster driver sound that people like with Fostex.
https://www.emu.com/teak.html
Either that or try to find an old Denon AH-D5000 on the used market, but those are probably getting hard to find in good shape.