They’re different. Much wider soundstage and better imaging, more natural, less harsh of a V, more balanced overall. The best way I can describe the sound signature is “larger than life”. It’s definitely the sound that will make an unprepared person go “Holy shit!” when listening to it for the first time. The 900 excites you with the bass and gives you a brainquake with that impact, the 909 excites you with the whole range and gives you goosebumps because you never thought your favorite track could sound that great. However, due to the open-back nature, some of the bass impact is lost — only the impact, not its quantity or quality, or depth for that matter, it’s still very articulated and precise with no mud whatsoever. It sounds amazing. Granted, there will be some sound bleed, you’ll definitely hear it if someone’s listening to it next to you — but if you’re like me (and I have a smaller head) the 900 will bleed a lot, too.