Richter D sent me his Mobius.
He does hear the hiss, I don't.
I do NOT hear it on his Mobius too.
I measured both and they are, within extremly small margins, identical.
So there is no difference, it's only, he has a more sensitive hearing than I have.
Noise numerically:
Mine (#1 in the plot): L/R 20,0 / 19,6 dB SPL Aeff
Richter D (# 2 in the plot): L/R 20,2 / 20,3 dB SPL Aeff
Neglectable differences, just my right driver is quieter (see plot) which makes for the tiny difference.
Here is the spectrum analysis, and below the frequency response.
As you can see, all the differences in the noise spectra in the 4 cups are reflected in the frequency response, so the electronics are 100% identical.
There are only acoustic driver differences, too small for being responsible for the difference in recepted noise.
Noise spectra A-weighted (Bruel&Kjaer Artificial Ear 4153, 4165 capsule):
Frequency response (Bruel&Kjaer Artificial Ear 4153, 4134 capsule):
Another interesting result: left and right differ in the amount of bass.
This is due to different acoustic properties of the left and right cup construction.
AUDEZE should fix that by a compensating EQ with the next firmware update.