I promised @Ermos a short review… The Stealth is on duty for a couple of weeks now. No way am I going back to the Focal Utopia - it will serve honorary duty on long trips. First I was a bit hesitant as the Stealth did not transport what I would dub colors of music. It was like looking through glass. The Utopia is good at rendering colors. By that I don’t mean yellow or red. It’s more an analogy or concept to describe an undescribable listening experience.
After reading all the enthusiastic posts here I was underwhelmed by the upgrade from Bartók + Vivaldi Clock to Vivaldi Clock + Upsampler + DAC. Not anymore.
I received the adapters for the Double Helix Prion4 balanced headphone cable so the Focal connector becomes one suited for the Stealth. Now that this is (re-)burning in … I’m really excited. Sonic and musical bliss. 
Things are still stumbling over each other somewhat, but that goes away with more burn in.
Listening with the Stealth at times borders on spooky. The music is just there - no coming into being.
The music fills the whole skull. Previously I was used to a rather flat plane between the ears. And then the ever evolving structure of the music becomes so readily apparent. The inner meaning remains beyond words but becomes comprehensible. All the musical texture gets palpable and musical timing - which is so important - is excellent and maybe a bit too precise as in strict. But then I came from an endgame Naim system and timing was their forté. The presence of the music is quite in your face without becoming bothersome. This is what I like and strive for - being immersed in the music. If one prefers the observer perspective - maybe this is the wrong headphone.
The closed design is great for bass which for my musical taste is important. Its punch is maybe not as organic as the Utopia but it reaches deeper.
One quibble - wearing the headphones can uncomfortably press on the head after a while. The cans are not too heavy but the padding is thin.
I know the Utopia very well. And in this setup it was the bottleneck (and the digital cables which were Van Damme, are Oyaide now and will be Black Cat Tron soon). All our tastes and dispositions are different as our thread with the images of the dCS gear at home shows so nicely. That said I don’t recommend the Stealth outright. It is definitely worth the effort to arrange a test listen.