Unfortunately, I haven’t had a chance to listen to them modded, but I do know that quite a few of current basshead recs are just officially modded Fostex models, with the T50RP being a rather popular choice. Would be an interesting experience for sure! Unfortunately, not many people have the desire and/or skill to mod an already pricey headphone, but from what I’ve heard, Fostex are pretty user-friendly in that regard.

My TH900 also has original TH610 pads on it, and sounds much more to my liking like that.

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.

Fostex TH900 (Mk I and II, they’re the same, but Mk II has a detachable cable) are so far the best I’ve found and one of the two pairs along the TH909 that has scratched the bass itch. Yes, they’re pretty up there in price if you buy retail, but you can get them either used (specifically the Mk I) for about $600 or on a sale (now is the time if that’s the case). The lower tier models weren’t it, so you can’t really save by buying a cheaper Fostex. No idea about the most recent release though, haven’t had a chance to listen to it yet.