What I like to share is, that everything works perfectly with linux (Ubuntu, Debian, 18.04) and with the ipad (air2).
I had almost 10 different audio-interfaces in the past, but none of it was so easy to configure. And no other had this audiophile quality.
I use the device only in CC mode, so it is recognized with my laptop and desktop running linux and with my ipad, running ios.
However, to other users in the same situation: there are two "tricks" I had to perform, that are documented elsewhere in this forum and in the manual:

1) starting the device in CC mode: push select and DIM on the device when it is powerd up to change between CC mode and standard mode
2) use "copy"-mode to route the output to the headphones: use "OUT"-Button to select OUTPUT Ch. 1/2, push "SET-Button" so that 3⁄4 Ch 1/2, Phones and Opt are lit. That means, Copy mode is active.

That was all that was necessary!
Now I tested the device with different configurations of attached inputs and outputs, and I never, never had a better sound. I mean, I am not a professional audio guy. So I do not have regular access to high quality, very expensive audio stuff. But for me, this babyface is worth every penny.

However, I am just a happy customer, I think.
But I just felt the need to share it. Since for me it is important, that I have a device, that is independent from certain software companys like microsoft or apple, that works with every hardware and with every every operating system. And this is rare to find. I found it! Greetings to all other linux.users!