It does not seem very practical to me to connect the FF400 in stand-alone mode via ADAT to a Digiface USB for the following reasons:
1. the FF400 has only one ADAT port, so only 8 channels can be transmitted in single speed (44.1/48 kHz).
2. the remaining 3 ADAT I/O ports of the Digiface USB are then without a concrete application. So you pay €349 but are using only 1 of 4 ADAT I/O ports.
3. you would still need a firewire connection so that the FF400 remains manageable.
In my opinion, it would be better to sell the FF400, with some luck you can get still a quite nice price for it and
use that money plus the €350, that you wanted to invest for a Digiface USB, to buy a new USB based interface instead.
Maybe Babyface Pro FS (€680) or Fireface UC (€824) or Fireface UCX (€1073).
What type of I/O ports do you need and how many of them ?
My Excel comparison sheet can help you to find the proper interface:
https://www.tonstudio-forum.de/blog/ind … B-MADIfac/
Excel File: https://www.tonstudio-forum.de/index.ph … 0-08-xlsx/
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13