Hi Joel (?), how are you ? My answers to your questions and two cost efficient proposals:

to 1) I assuming you mean a Rosetta 800 with 2x ADAT I/O (IN and OUT) ?

to 2) yes possible as the Rosetta 800 has also ADAT IN to receive clock. This would be the least expensive solution.

Digiface USB <----ADAT1 IN--------------   UCX II ------------------AN------------> Main Out / Phones_1
                                                                 "       <----------------AN-------------  Dangerous Music 2 Bus+
                                                                 "       ------------------AN--------------^   ^   ^
    "                ------ADAT1 OUT------------>  "   (clock slave)                                   |    |
                                                                                                                            |    |
    "              <----ADAT2 IN-----------------  Rosetta 800 #1-----AN-------------------+    |
    "                ------ADAT2 OUT-------------> "   (clock slave)                                        |
                                                                                                                                 |
    "               <----ADAT3 IN-----------------  Rosetta 800 #2----AN------------------------+                   
    "                ------ADAT3 OUT-------------> "   (clock slave)
                                                                                              (AN=Analog)

to 3) every RME recording interface supports loopback operation. So you can use loopback recording with the Digiface USB.
This is possible on each type of HW output, not matter whether this is an analog or digital HW Output.

Only as a note, no issue for you: the implementation of loopback for the Digiface USB is post-fader (+)

(+)The loopback function in TotalMix FX is implemented either "pre-fader" or "post-fader", depending on driver and hardware support. The "pre-fader" implementation allows to mute the HW output without affecting the loopback operation. This is useful, for example, to mute connected active monitors without affecting the loopback operation, otherwise you would have to disconnect the cables of the active monitors, which of course would not be so convenient.

See also: https://www.tonstudio-forum.de/blog/ind … ion-EN-DE/

Alternative solution - RayDAT (PCIe)

If a PCIe card would also be possible either to be used in your computer or through an Thunderbold PCIe expansion cabinet (e.g. those from Sonnet):

The RayDAT is purely digital and more versatile as it has more very useful I/O ports:
- 4x ADAT I/O (ADAT4 also supports optical SPDIF)
- AES
- coaxial SPDIF
- 2x MIDI
Even a word clock expansion card is (optional) available for the RayDAT.

You can even use two or more cards. Up to three are tested by RME and are officially supported by the driver.

But the driver could even use more, as RME stated in this forum. But this has not been tested and also much depends on whether you PC is able to drive more cards with more channels without issues.

Clock can be distributed by internal ribbon cables, which is very flexible, as this saves one WC expansion card per RayDAT and saves also PCIe Slot capacity in your computer case.

To sum up
Now you have two cost efficient solutions, assuming that a >€7000 MADI based solution is not what you are out for ...
- one based on a very inexpensive (Digiface USB) and
- one based on a PCIe interface (RayDAT) which gives you a very solid set of I/O ports and optional WC

Both recording interfaces are currently available on the market. The RayDAT I own myself, it's a real phantastic card.

Options - ADI-2 Pro FS R BE

In contrast to the proposed quite expensive MADI solution in post #2 this would even leave room to e.g. integrate an ADI-2 Pro FS R BE to enhance your TM FX monitoring section for active monitors and phones. The AES port of the RayDAT could e.g. be used to directly attach an ADI-2 Pro FS R BE at any time later.

See also:
https://www.tonstudio-forum.de/blog/ind … ses-EN-DE/
https://www.tonstudio-forum.de/blog/ind … our-Setup/

Options - ARC USB

BTW .. You could also think about adding an ARC USB, as with TM FX interfaces the ARC USB does not need to be attached directly to a recording interface (which is only possible with UCX II, UFX II, UFX+) you can also attach it to the PC and the TM FX multiplexes the communication through ARC USB and the recording interface, no matter whether Digiface USB or RayDAT (and all the others back to 2001).

Options - MADI based solution (high cost)

Of course there are MADI based solutions possible. They are very nice, I use one myself and gained a lot of experience with it. I also tested the very nice HDSPe MADI FX card which is very uniq with it's three MADI busses and a full implementation of the FX chip on it. It also has a resource optimizing driver, which only allocates cpu/driver resources when channels are being used (in groups of 8). But MADI has a certain cost impact. If you want to know more about it, then let me know.

Final thoughts

With the two proposed solutions above (Digiface USB or RayDAT) you can save a lot of money and looks like a sufficient solution for you that fits your requirements.
And you can even re-use your existing devices or even get another Audient on used market as you mentioned.

If possible get the RayDAT. I own this card myself and can really recommend it. Highest flexibility and scalability with this card and even word clock if you should need it later for external devices without digital input which need WC for this reason.