I owned a 780D v2 for a while and really enjoyed it. It’s a beautiful device. The chassis design is distinctive and striking in its appearance - not just another generic black box in the rack. Its performance specs are state-of-the-art. It sounds absolutely heavenly. I found it to be easy to use and always reliable.
I’m aware of the 680D, but have not auditioned it, so I can’t offer any first hand perspective on it. I read an interview with Dominique Poupart, Simaudio’s Product Manager, in which he discussed the technical differences between the 780D and the 680D. The 780D uses two converter chips in dual mono mode, where the 680D uses one. The 780D uses a more expensive “femto second” clock, where the 680D uses a lesser clock. The SNR, THD, and IMD specs are very slightly better in the 780D. While a notable engineering achievement, I doubt I would be able to hear a difference between them. At any rate, the 780D is still their statement piece and that means something to many audiophiles.