If so, I stand corrected.
Ok, now some enlightenment: The measurements have been carried out with symmetric and asymmetric cables. So I picked up first a short symmetric connection and then quickly an asymmetric cable flying around.
Now the Fireface has symmetric inputs and outputs which require jack plugs. The symmetric connections require stereo jack plugs and the asymmetric connection uses mono jack plugs.
So the noise difference in my previous comparison is caused by connecting the cold pin to the ground pin.
Here is a comparison of a short and long symmetric cable:
And here is a short and long asymmetric cable:
The noise floor for the asymmetric connection is about 10 dB higher. But the difference between short and long cable is much less and possibly neglegible.
And the world is back in order