Your description is unclear. In the topic you talk about ADI-2 DAC FS and ADI-2 FS (2 different devices), but in the text you talk about FF800 and 2x ADI-2 DAC FS (not ADI-2 FS anymore).

Please correct the posting / topic as needed.

From your description, it is also unclear, how you intend to connect the devices to understand your question.
Please provide a drawing.

Is the intention to perform such a setup, see below?
If yes, is the question, whether ADI-2 FS provides better D/A conversion quality compared to FF800?

PC
|
FF800---ADAT---[clock]--->ADAT---ADI-2 DAC FS---AN1/2--->Active Monitors
    \                                                                 \---AN3/4--->Headphones
     \
      \------ADAT<--------------ADAT---ADI-2 FS---AN<-----analog device
       \-----ADAT----[clock]--->ADAT---/

Besides the question, I would simplify/streamline the setup.
If I understood correctly, you have an ADI-2 DAC FS now.

To get higher quality AD into the setup, I would sell the ADI-2 DAC FS and buy an ADI-2 Pro FS R BR or even ADI-2/4 Pro SE instead.
By this, you have the AD in one device and additionally, ADI-2 Pro FS R BE and ADI-2/4 Pro SE provide you with even more interesting features.

PC
|                 
FF800---ADAT<---------------ADAT---ADI-2 Pro FS R BE---AN<----analog device
         \----ADAT----[clock]--->ADAT---/                           \---AN1/2--->Active Monitors
                                                                                    \---AN3/4--->Headphones

Maybe 1st of all something which I do not understand.
Why are you using FF800 and ADI-2 DAC FS separately?

I ask because you can integrate the ADI-2 DAC FS nicely into your setup.
By this, you would have FF800 with TotalMix to have nice control about routing.
You would have no loss in audio quality sending audio to the ADI-2 DAC through the ADAT port of your FF800.

Even if you use the ADI-2 DAC FS kind of standalone in such a setup you can still keep it connected through USB to be able to perform firmware updates or to remote control it using the new ADI-2 Remote software.

How to integrate an ADI-2 DAC/Pro into your environment you can see from this blog article.
https://www.tonstudio-forum.de/blog/Ent … our-Setup/

May I ask in which country you live in? If you do not want to tell in public, you can send me also a private mail.
Not sure why you exclude selling through eBay. Maybe reverb.com suits your demands better?

Selling RME equipment is usually easily possible and you get good prices because of quality and long driver support. Depending on the device, age, condition you can expect around 75-80% of street price for newer devices.
For an ADI-2 DAC FS you could get around 75% of street price which increased from €1000 before corona/chip crisis to now around 1200. So I would guess you could get a price between €800-900, especially if the devices in your country may not be good available ... For older devices like an FF800 in good condition you might still get around €400-500. I also saw higher prices, but it is difficult to say (didn't watch this market) whether the sell offers also reflect what people are willed to spend

There are many people looking for used RME gear in case they can not effort new devices. Which makes selling attractive and "win win".

You could even think about getting an UFX II, as it got the converter of the ADI-2 Pro FS and you get besides nice features also the new Room EQ.

Selling ADI-2 DAC FS and FF800 might give you at min €800+400=€1200 maybe even a little more.
A new UFX II costs €2099. Just a thought ...

The less expensive solution for you might be to keep FF800, sell ADI-2 DAC FS, buy ADI-2 Pro FS.
By selling ADI-2 DAC FS you get around €800-900.
Then you only have to pay additional €800 for an ADI-2 Pro FS R BE.
An ADI-2 FS costs around €700 but has less features.

My opinion is to sell the old stuff as long as you get some money for it and as long as the devices still run and do not become defective. At a certain point in the future there will be no interest anymore into getting used FW devices.
You should sell FF800 before reaching this point to get at least a little money out of it.

But this would mean you might need to sell maybe also ADI-2 DAC FS to better cross finance an UFX II.
With this device you get excellent ADDA conversion, you might only miss certain features of the ADI-2 DAC.
But well .. wait 1-2 years, save some money, and add then an ADI-2 DAC FS or ADI-2 Pro FS to your UFX II.

There is no difference in quality by sending digital audio via USB or via ADAT.

I am also using such a setup without any issue, even to two ADI-2 Pro FS R BE (studio / HiFi corner).
UFXIII----ADAT1--------[SPDIF]------ADI-2 Pro FS (Studio)
        \----ADAT2--------[SPDIF]------ADI-2 Pro FS ("High-End" HiFi)
         
You can validate this yourself by performing the Bit test which is an end-to-end test for lossless audio transport.
From the audio player up to the DSP of the ADI-2 DAC FS.
Ensure that the volume settings on the audio player (or DAW) is set to 100%.
If you lower the output volume then the Bit test can not succeed.

Also check, that clock synchronization is proper.

FF800 should be set to clock master (clock source = internal), see driver settings
ADI-2 DAC FS should be set as clock slave (clock source = ADAT). Easy check via "state overview" (last line, Clock Source..)

The design of the ADI-2 DAC/Pro series of devices is in a way, that the final D/A conversion will always be performed using the internal FS clock no matter whether the ADI-2 DAC/Pro is clock master or slave. This is a unique design feature of these devices.

When comparing sound, then ensure, that you listen at the same volume level as for our ears louder sounds better.
Difficult might be for the comparison when you don't do it blind and if you can perform quick A/B.

You can read about the Bit test in the manual, ch 31.14.

In short, you download the short test files in WAV format, there is one for each sample rate (free of charge)
You play them back in your player.
ADI-2 DAC FS has an internal detection which automatically recognizes the test pattern.
If the test pattern has been transferred without any alteration (not digital bit of digital audio stream changed),
then you know that the audio transfer succeeded "end-to-end"

PC-----FW400-------FF800-----ADATorSPDIF---------ADI-2 DAC FS
Player.............................transport.........................DSP
END---------------------------------------------------------END
                   "End to End" test from player to DSP

BTW .. the URL to the Bit test in the manual is not correct
https://rme-audio.de/download/bit_test_wavs.zip

Nice, that you could validate this (better) setup for you. You can keep USB connected though, for firmware updates and to be able to use ADI-2 Remote for remote controlling the unit.

Regarding A/D conversion quality, ADI-2 FS vs FF800.

RMEs converter can be characterized to perform a "transparent conversion" without adding any mocho to the source signal.

15+ years newer converters are faster and will be better from the technical specs.

There are some people claiming the newer converter sound better and deliver a better resolution of the sound material in terms of bass, room resolution.

But it is difficult to predict how more it brings for your use case of performing A/D.

All that I can tell is that the ADI-2 reference converter like ADI-2 Pro FS (older model) performed quite well in the mastering converter shootout. There the older model (ADI-2 Pro FS) was compared to HEDD and Lavry AD converter.
See also my blog article: https://www.tonstudio-forum.de/blog/Ent … ses-EN-DE/
This YouTube video from RME: https://www.youtube.com/watch?v=doHG32aXBDY
In the comments section of YouTube video you find the URL to download the original WAV files (as YouTube changes audio).

How ADI-2 FS compares to ADI-2 Pro FS is also difficult to predict.. You need to try it out yourself.