I own both cans so I can compare. Detail retrieval is similar but the presentation is different. Slightly better on Abyss but not by much at all. Treble is more pronounced on the Abyss so you have the impression of more details. When I listen to the abyss I hear more reverb on female vocal track for instance or pianos. Soundstage is without a doubt wider on the Abyss but not as wide as an HD800 or Arya. It also really depends on the music you listen too. For rock I think I prefer the Final, for Jazz and classical the Abyss. Both D8000pro and 1266 TC have their merits but if you care about soundstage neither of them is the king. Bass is presented differently. On the Abyss you get a rumble that is almost physical (that is when you wear the pads with a small gap behind the ears). You can “feel” it on a drum kick or some EDM with pronounced sub bass (doing it right by Daft Punk is a great test track for that). On the Final I think bass is somehow slightly more clear but it is not as physical (I think when you get the sub bass you lose a tiny bit of details). If I were to buy only one I would buy the Abyss to scratch the itch as it is technically the better headphone and you would always wonder how it sounds. Owning both, I sometimes find the Abyss a bit fatiguing and I like to listen to the final for longer sessions.
In my opinion, this is mostly due to the fit as the abyss doesn’t really disappear from your head as the pads feel quite hard. The final feels like a normal headphone with very comfy pads. Both are on the heavy side but the abyss is heavier. I don’t find neither too bad for that.
I also own both and while I largely agree with the above comments i do have a slightly different perspective and thought I would share my viewpoint. I find the Abyss to be more crisp sounding overall and somewhat tunable to individual tastes. I find the D8000 Pro to be a little more natural sounding in terms of tonality and timbre. As for bass, I think they both have similar impact and quantity, at least the way I have them configured, but I do prefer the slightly less rounded bass of the Abyss. Also, I agree that the Abyss has more treble, but for me I preferred what I perceived as the more accurate treble of the Final Audio treble. As for soundstage, I haven't really noticed one headphone feeling limiting compared to the other, For whatever reason, my ears which are sensitive to soundstage generally, do not hear that much of a difference - at least not to any extent where I am dissatisfied. In terms of detail retrieval, I actually think the d8K pro retrieves more detail, not by much mind you, but by a little. It is especially noticeable to me if I hear distortion in the mid to upper frequencies of music (example: 'Take you to Hell' by Ava Max as found on You Tube). I hear it much more clearly / pronounced on the D8K Pro than I do on the Abyss. I do agree with others that the Abyss may be preferable for classical music and the D8K pro on rock. For me the Abyss, just sounds more realistic/refined on orchestral - maybe that is due to soundstage, not sure?
Here is the other thing, and this is probably just me. If I want to just listen and relax I actually find the Abyss to be my go-to. If I want to be wowed and or listen intently and intensely, I go to the Final Audio. BUT, the Final Audio, for whatever reason, makes me think more about the equipment.
Both of these headphones are absolutely wonderful and you cannot really go wrong. In fact, I was surprised they are more similar overall than I expected. I do agree that the D8000 Pro feels more like normal headphones. The Abyss seems kind of stiff, but not uncomfortable at all. Both are heavy and I could see both being uncomfortable for some.