I haven't listened to, or know the TH909, but quite often the open back headphones that have more bass quantity tend to be less open, something between fully-open and semi-open, also pad material plays a role, leather/faux leather tends to be bassier compared to velour/textile earpads.
If you are after bass and are willing to EQ, Fidelio X2HR can be had for cheap and have outstanding performance/price ratio, or you have plenty of planar magnetic alternatives to choose from.