Assumed you want that your UC is clock master, then you need to use two ADAT cables.
The one that you need to get audio from the converter.
The 2nd one to transport (besides audio data) the clock to the FF800 as clock slave.
If your converter has no digital inputs, perhaps because you use a preamp as line in's,
then you need to use WC (word clock) or you need to make the FF800 the clock master.

PC----USB----UC---ADAT OUT----------------------------->ADAT IN---FF800
                       \----ADAT IN<----------------------------ADAT OUT---/
clock master                                                            clock slave
clock source "internal"                                            clock source "ADAT" or "optical"

optional Word clock cabling:
PC----USB----UC---ADAT IN<----------------------------ADAT OUT---FF800
                      |-------------------------------------------------------------| WC
                  master                                                                    slave

Not preferrable, because then you need to change the sample rate on FF800 and this needs a permanend FW connection.

PC----USB----UC---ADAT IN<----------------------------ADAT OUT---FF800
                            slave                                                             master

If you have no FireWire on your computer, then it is impossible for you to manage the FF800.
At least you need FW400 (or FW800) to preconfigure it one time for
- proper clock settings (best as clock slave) so that the FF800 can follow clock changes
- proper routing

The better solution is probably not to get such an ancient FF800 at all.
Firewire is not officially supported anymore by Apple and Microsoft.
You can get it still working, but the question is for how long or when such an old unit will fail.

IMHO two better choices

a) get a preamp (as you told already), but then you have two devices and not the quality of RME

b) get a recording interface with the sufficient I/O, then you can get maybe everything in one box which is nice
    especially if at any time you would like to record in double speed (88.1/96 kHz).

With double speed, you will have only 50% of the channels over ADAT, because of port "bundling/multiplexing".
As you have a UC with only 1x ADAT I/O this is a bottleneck here only 4 of the 8 channels over one ADAT link.

Therefore, the better solution for this might be to get a bigger recording interface which has
- the required analog I/O already on board and
- which has two ADAT ports so that you are more flexible in connecting
   either two external devices at single speed or
   one external device (preamp, AD/DA converter) at double speed without loosing ports

I do not know your budget situation, but would ONE FF 802 FS or UFX II also be possible and sufficient for you?
You could use the UC to cross finance this solution.

You can stay at recording in single speed, then 1x ADAT I/O (IN and OUT) is sufficient to you.
I only wanted to mention it to you to give you an overview.

> I need the firewire connection only for the initial configuration, right

Yes, because the FF800 supports stand-alone operation, see manual

"The Fireface 800 has an internal memory to permanently store all configuration data. These are:
- Settings dialog, sample rate, clock mode Master/Slave, configuration of the channels and the digital I/Os.
- TotalMix The complete mixer state."

If you have a Mac with thunderbolt, then I remember from forum postings, that with a combination of thunderbolt cable and adapter the connection of such a device might be possible. But if I recall correctly, then it has to be Apple adapters, the other cheaper ones won't work.

Then I think I would get the FF800, theoretically you could even try to use this as your main recording interface and connect the UC behind ADAT.

Maybe somebody with Apple knowledge can help here.

I am thinking about something like this:

Apple---TB-----TB FW Adapter---FF800--ADAT1 OUT------------------>ADAT IN--- UC--->Active Monitors and Headphones
                                                       \----ADAT1 IN--------------------ADAT OUT---/

                                                     ADAT2 completely free, switchable from ADAT to optical SPDIF
                                                     This way you could add later even an ADI-2 DAC for your monitoring section
                                                     to get a better converter

By this, you would have a FF800 as 56ch interface, the former flagship interface before UFX and its successors
with the following feature set:
    10 x Analog I/O
    1 x Stereo Headphone Out
    2 x ADAT I/O or 1 x ADAT I/O plus 1 x SPDIF I/O optical
    1 x SPDIF I/O coaxial
    1 x Word Clock I/O
    1 x MIDI I/O
    4 x Mic/Line Inputs with Preamps
    1 x Instrument input adjustable
    2 x FireWire 800
    1 x FireWire 400
    optional: Time Code Module
    TotalMix

Just a thought, the forum colleagues with Apple might help here to find the best solution.

On the other hand, combination of UC and a preamp or adda converter could be the more solid invest.
USB is definitively more future proof ....

The combination of UC and ADA8200 is the cheapest solution, and you get the best buck for only little money.
The 8200 can not only be used as a preamp but also as AD/DA converter.
You can route from ADAT to its analog outputs… making it flexible in use.
So it can also easily get clock via ADAT, so you need only two TOSLINK cables, no hassles with Word Clock cabling.
It has dip switches on the back to get clock from ADAT to make ADA8200 to a clock slave.

Awesome, thanks for the ver helpful answers! I think I will look for a FF800 at a good price or otherwise go with an ADA8200 alternatively.

Hi
Yes, in your matrix window I can see the analog inputs aren’t routed to the ADAT outputs. If you want to use analog inputs 1-8 on the FF800, you have to route them to ADAT outputs 1-8 (so output 13-14 on top or ADAT 1/2, ADAT 3/4, ADAT 5/6, ADAT 7/8 on bottom). Also make sure to disable (close faders) the analog inputs going to analog outputs!

EDIT: like this: