1) First I recorded RHA30 using HD215 as microphone:
2) And also Momentum using HD215 as microphone:
3) Next I recorded RAH30 using Momentum as microphone:
4) Next I recorded Momentum using RAH30 as microphone:
On 1) we see dip at 12Hz, and on 2) we see bump at 10Hz, then on 3) we see that bump again as artifact introduced by using Momentum as microphone, and on 4) we see that dip of RHA30 and bump of Momentum at 10Hz they added up to make straight line.
Looking at 1) and 2) we can see that noise floor is 10dB lower for RHA30.
Neither RHA30 nor Momentum can go below 40Hz without distortion. The difference between 1st and 2nd harmonics is 30dB for RHA30 and 25dB for Momentum.
Headphones were driven by T744, which got it's signal through toslink.
I personally think though that harmonics of 40Hz sound on Momentum are more prominent when you listen, because of closed design and resonance. While RHA30 open design is free of such resonance the distortion appears more because of physical properties of the driver, which at 40Hz is pushed to its limits.