Hi
In my opinion you misreaded something. Optical ADAT (8 channels), AES over XLR and SPDIF over RCA are independent from each other and work in parallel.
It’s only, that the optical i/o ports can also be used for SPDIF signals if needed.

Hi, thank you for the answer Oli! I was checking TotalMix for that, I guess what's puzzling (and that's in both manuals, ADI-2 FS and UCX ii) is that if you adapt the SDPIF/RCA to use it as AES, it takes over the first 2 ADAT outs and changes the ADAT ports to SPDIF consumer?

That section of the manual is very puzzling, could really be I just misinterpreted the whole thing and as long as I do RCA to RCA and SPDIF everything coexists? Are you using a similar setup?

No, setting the so called 'ADAT' - what means the optical ports - to SPDIF, you have two further SPDIF channels instead of the eight ADAT channels. Both the XLR AES/EBU and the RCA SPDIF stay untouched from this!

So it is either

8 Optical ADAT +
2 RCA SPDIF +
2 XLR AES/EBU

or

2 Optical SPDIF +
2 RCA SPDIF +
2 XLR AES/EBU

Again, what you have in hardware on the UCX II is not replacing each other. You can use all of the three digital ports (optical + RCA + XLR) in parallel. There is no channel loss.
To my knowledge it’s indeed the case on other interfaces where you have to choose between optical or coaxial (RCA) SPDIF because there are only two SPDIF channels possible in total. But not on the UCX II!

The coaxial / RCA, so called 'SPDIF' connection can also be used for AES protocol (using a corresponding cable with RCA plug on one side and XLR on the other.

And also the digital XLR, so called 'AES/EBU' can be used for SPDIF protocol.

So, in addition to my list above it could even be:

2 optical ('ADAT') SPDIF +
2 RCA ('SPDIF') SPDIF +
2 XLR ('AES/EBU') SPDIF =
6 SPDIF channels in total.

Of course in this case you would lose the eight ADAT and the two AES channels.

But also:
2 optical ('ADAT') SPDIF +
2 RCA ('SPDIF') AES/EBU +
2 XLR ('AES/EBU') AES/EBU

Or further mixes of the three connectors / connections.

One of the few downsides of the generally great written RME manuals is the fact, that for the digital connections some confusing synonyms are used:
'ADAT' is used instead of 'optical',
'AES/EBU' is used when the digital XLR connectors are meant.
But this is a general problem, names of digital audio protocols and hardware connectors are often mixed because there also are common uses…

Oli THANK YOU, now I understand everything after almost two years of imagining a bunch of the digital connections were mutually exclusive RME people could definitely take a look at this conversation and do some updating of the manuals... In any case, now you've just shown me the most obvious path forward, 2/2 extra I/Os with greater dynamic range and less noise, combined to 8/4 analog I/Os and 8/8 optical.

That nets me 14 outs (2 mastering quality) and 18 ins (2 mastering), with 2 sets of monitors connected to the whole thing.

Sorry, but you are probably mistaken.
I could imagine that you have completely disregarded basic methodologies for comparisons here.

Just a few examples:

• Rewiring and reconfiguring takes time, our brain can't remember such sound details for that long.

• Such A/B comparisons must be made at the same volume level because louder always sounds better to our brain.

• Then you should also use blind tests to rule out a certain expectation (brains "sadly" work with such a bias).

From a technical perspective, I think it is extremely unlikely that there will be any differences in sound (if you take the above-mentioned things into account and carry out valid tests), as

• Both devices have FS clock and

• use the same "SteadyClock FS" technology as clock "slave" for jitter attenuation.

• Another point: you mention the Neumann is being connected through AES. So, no matter whether UCX II or ADI-2 FS are clock master or slave, the Neumann is always the slave to an FS clock, there is no difference.
  

Incidentally, for such discussions, I am missing something like a small drawing of the two cabling scenarios you are comparing here. With details of who is the clock master and where the active monitors are actually connected and how (analog or digital) so that it is clear where (D/A) conversions are taking place.

EDIT: Furthermore, a brief description about test methodology to exclude the above-mentioned errors which only lead to wrong and maybe also confusing results and discussions.

Thanks, I bring this in a different order which is easier to see and remember for me

Option A - so the current scenario seems to be this

PC
|
UCX II---AES--------------------->AES---Neumann KH750---AN(L/R)---Satellites (Speaker)
||      \
||       \---ADAT------------------->ADAT---Motu 8 Pre es
||         \---ADAT<-----------------ADAT---/
||
|| SPDIF (coax)
||
ADI-2 FS (cm)<---->Compressor

cm=clock master

Option B - the other one with UCX II as clock master

PC
|
UCX II(cm)---AES----------------->AES---Neumann KH750---AN(L/R)---Satellites (Speaker)
||      \
||       \---ADAT------------------->ADAT---Motu 8 Pre es
||         \---ADAT<-----------------ADAT---/
||
|| SPDIF (coax)
||
ADI-2 FS <---->Compressor

To be honest, there is not much of a difference. Whether the UCX II sends out its clock signal or one as clock slave "purified" by its own SteadyClock FS circuit doesn't bring a significant change.
All is still based on jitter free FS clock technology:
- be it a clock signal from UCX as clock master with clock signal from its internal FS clock or
- a clock signal from ADI-2 FS, refreshed by UCX II (slave) and refreshed by SteadyClock FS technology.

Please consider that we had years ago already a discussion, whether there is a difference between ADI-2 Pro and ADI-2 Pro FS and the conclusion was that it most likely only a measurable difference if at all.

And here in this case you have even SteadyClock based tech in place, no matter whether ADI-2 FS or UCX II is the master.
The Neumann is always the client.

Now look at this RME video, how well the Jitter Suppression of SteadyClock FS works:
https://rme-audio.de/steadyclock-fs.html
https://www.youtube.com/watch?v=Ti0aHW-zYcs

Therefore, I have doubt that you can hear any difference (sorry).

But you are free to choose whatever you prefer.

Maybe only one tip, for operational reasons it would deliver to you more advantages to use the UCX II as clock master.
Because then you can determine the sample rate by the application, and the slaves can learn the clock from the master.
Or in other words, you have most likely only to select the sample rate in the DAW or in the Music Player and not additionally at the ADI-2 FS as clock master if the UCX II is the master.