The signature pros were intended to be the next step up from the edition 8. The "carbon" edition looks like they just changed the cosmetic appearance of the unit.
The Ultrasone Edition 8 and Signature Pros really came from a headphone era when a "high end" closed headphone didn't exist. The last few years of new headphones kind of make them obsolete at their price point. There's a lot of competition that outdoes them, so if you want an "upgrade" options do exist now.
I don't agree that the sigpros were intended to be an upgrade. The Edition series were meant for a different audience than the sigpros. The Ed 8 is made for more of an audiophile sound. I think the sigpros are more balanced but don't have the clarity, soundstage or instrunmsnt separation. I love them both and go back and forth on which I prefer, My ed8 needs a new plug put on the cable though and I haven't been able to find anyone local to do it😁 I think if push cam to shove I would take the ed8 due to comfort and sound but the sigpros seems indistructable and is more portable and easy with the single cable out.