Adi 2 pro fs black to digital monitors

You don't mention what digital inputs your speakers have (or which speakers)

The main setting you need to make to use it as a digital preamp is to switch the "Dig. Out Source:" setting to "Main Out". Then the digital output will have all the DSP processing applied to it, including volume, B/T, PEQ, dynamic loudness, width, crossfeed, polarity, M/S.

So Adi can stay connected as it is to receive clock from USB Digiface by optical In/Out ??

Then configure AES out do the monitors ??

PC
|
| USB
|
Digiface USB [cm]---ADAT1 OUT---------->ADAT IN---ADI-2 Pro FS R BE [cs]---1/2---Act Monitors
                                                                                                             \---3/4---Phones

Digiface USB can act as clock master and send out the clock signal on every digital output.
ADI-2 Pro FS R BE will be configured as clock slave receiving the clock through its optical ADAT input.

Although the ADI-2 Pro FS R BE can recognize ADAT protocol (it uses ch 1+2 out of the ADAT stream)
I would recommend configuring the ADAT output of the Digiface USB to use SPDIF protocol,
as this is a 2ch protocol for 44.1 - 192 kHz without sample multiplexing.
By this autorecognition of sample rate between 44.1-192 kHz is possible at the reference converter.

The reference converter is, with its SteadyClock FS implementation, the perfect clock slave.
Although being a slave, the internal FS quartz is being used for the D/A conversion.

Little addition to what @ramses and @unplugged already said:

Set ADI-2 Pro to (manual chapter 17.3) AD/DA Converter Mode.
Have a look at the very informative block diagram there.

As you can see, the digital outs AES, Optical and Coax are always linked together, can all be used at the same time.
If you want to control digital monitor volume and EQ from ADI-2  (setting Dig. Out Sourcs: Main) this affects all three.

Some digital monitors have their own volume control built in, even offer a remote control, and quite often have room EQ etc.
It’s to consider if you’re better off using theirs.

What model of digital monitors do you get?

Some, like some Neumann models, ask for non-standard interfacing (AES on BNC connectors).