And thanks for the additional information on my ADI-2 didn‘t know that it can convert AD and DA at the same time. To be fair, I never planed to use it this way but maybe good to know for future projects 
(Thought it was not possible due to far less controls in comparison to the bigger units - but haven’t gone to all of the manual to be fair)

As an alternative to the DS, you can also look for the ADI-8 AE, which is technically like the DS, but has a few dB less of S/N ratio on the AD side. Nothing really relevant, I used one for years, it's a good converter. If you stumble across something called ADI-S or so, that's a DS as well. And there were Steinberg/Nuendo branded versions.

QS is the successor of the DS, it's not a "split".

Your ADI-2 should be able to to AD and DA as well, by the way... What makes you think otherwise?

Unclear what device you are speaking, lot of the RME devices are Pro.
I had ADI-8 QS and never had to do a setting again. I had a main power switch for the entire studio and the ADI-8 started to work and did converting AD / DA both way at the same time.
May you read again the user manual of the device you want.
Please use the exact product name. ADI-8 Pro for example is not even listed under legacy products.

It is the first device you'll find via google and also listed in the RME archiv here: https://archiv.rme-audio.de/products/adi_8_pro.php
As far as I remember the "Pro" this is the predecessor before the ADI-8 family split into DS and QS

I already read the manual but there is nothing in there that gives an answers those two questions otherwise I wouldn't have to ask.

I think the second question is answerd, maybe someone got the Pro unit and experience with it's behaviour. Otherwise I may ask one of the sellers if they experienced such a behaviour. I just read once about this "problem" in a dead thread of another forum... There was never mentioned if the unit may was broken or something like this.

I already got an ADI-2 and this one is only able to convert either DA or AD (depending on the settings), that's why I wanted to know if the bigger units can convert in both directions simultaneously. 

I wanna build myself different monitoring possibilities with the ADI-8 so I need AD & DA at the same time (signals coming from the mixer and going out to the monitoring speaker / headphone preamp and so on)  

The answer was very helpful! I hope there is still someone who can tell me something about my first question.

The UC has an ADAT IN and ADAT OUT port. So you can send and receive 8 channels via ADAT @single speed.

A Mic preamp performs typically A/D and sends the mic signals through its ADAT port out to the recording interface.

The other direction, to use the Mic Preamp as D/A converter is only possible, if the preamp is designed this way, that you can route audio from ADAT IN to the analog outputs.
Some Preamps have analog outputs, that can only be used as CUE OUT port, to mirror/copy the recorded audio from the corresponding mic input.

You can, of course, connect different devices, for example:
use ADAT IN to connect an external mic preamp and
use ADAT OUT to send audio to an external D/A converter.
Up to 8ch @single speed, up to 4ch @double speed, up to 2ch @quad speed.

Careful if the Mic Preamp is only connected to the ADAT input of the recording interface.
Then clock synchronization can only happen though Word Clock or you need to configure the preamp as clock master.

In terms of clock synch, check RME manuals and see my blog article here:
https://www.tonstudio-forum.de/blog/ent … ios-en-de/

I'm looking for a possibility to extend my Fireface UC's IO via ADAT and found some used ADI-8 Pro and ADI-8 DS units for sale but I have two questions left which I can't find an answer to.

1) I read that the Pro is forgetting all settings when powered off, is this true? This may be a dealbreaker to the Pro if I always need to dial in all settings while starting it up.

2) Is it possible to convert A/D & D/A at the same time? It would be perfect to use all ADAT In and Outs of my Fireface with only one additional unit. So can it do both or do I need one unit for each conversion?

I hope you can help me out with this.

Best regards