Curious. It seems that every response curve I've encountered over various headphones for SoundID Reference are tuned and take huge dips to go flat at 20 Hz.
And creating a custom bump at 20 Hz doesn't do much, looking at the resulting curve, SoundID Ref just neuters it.
Does no one here listen or record pipe organ music (16 Hz or lower with harmonics)? ;)
It's one of the reasons lately I've gravitated towards other means of headphone tuning.
Any reason for zeroing in on 20 Hz? Maybe it's in a white paper and I've missed it.