So my update:
I bought the iPhone 8 and compared to the iPhone 6 and again to the iPad Pro 10.5, then I bought the Audioquest Dragonfly Red.
These are my conclusions:
1. I think that the iPhone Adapter sounds a little better (= better soundstage and warmer) than the iPhone 6 headphone sound (yes, I know that tests claims something else!)
2. With the Adapter, both iPhone 6 and 8 sounds the same (= quality and volume)
3. The iPad Pro 10.5 sounds better than the iPhone 6 / iPhone Adapter (better quality and volume)
4. The Dragonfly Red improves the audio quality noticeably
5. Again, both iPhone 6 and 8 and iPad Pro sounds the same with the Dragonfly Red
Now I have to decide if I want to keep the Dragonfly Red or instead to buy the Mojo, which should be much better but also expensive (180€ vs 500€).
Cheers