I'd agree with
@Contrails with his LCD-3 Pre-fazor recommendation. In addition to that I'd suggest three of my own.
Focal Radiance - Definitely the darkest/warmest in Focal's line-up.
ZMF Atticus - Easily the darkest in ZMF's line-up. I really like dark leaning headphones and this was way too dark for me. I sold after two weeks of ownership.
Rosson RAD-0: Dark tuned - My Rosson RAD-0 is the neutral middle tuned option and it already leans very close to the LCD3 PF in bass tuning. I wouldn't call it dark but it's certainly not bright or neutral. Very warm. The Dark tuned variant takes the middle option and adds a lot more mid bass/sub bass and is the darkest planar i've ever heard.