Hi Wim,
regarding your question about RME UCX II and AES.

I gave some additional information in this context in the hope that it might be either of interest or maybe even useful for your setup and your planning.

Using AES, you can connect any other device with an AES port for transferring two audio channels with sample rates of 44.1–192 kHz. The transfer of audio data is in digital format and lossless.

With AES you have a balanced connection with higher signal levels like typical for studio use which supports longer cable runs. Usually, the design is transformer-based so that you have additionally a galvanic isolation.

With an adapter cable (RCA to XLR) and changing the transfer protocol to "consumer" in the driver settings, you can also connect devices with coaxial SPDIF. This gives you a 2nd coax SPDIF connection for this device if needed. Pinout for that, please see manual chapter 21.2.

You could, for example, connect one of the reference converters to enhance your monitoring with advanced quality and features.

How to integrate such reference converters, like e.g. ADI-2 DAC FS, ADI-2 Pro FS R BE or ADI-2/4 Pro SE, you can see in my blog:
https://www.tonstudio-forum.de/blog/Ent … our-Setup/

Just for the case you are interested, additional information about different models (except the latest, the ADI-2/4 Pro SE) you can find here.
https://www.tonstudio-forum.de/blog/Ent … ses-EN-DE/

This document explains/summarizes a lot about the basic cool features of those devices, and most if not all the information about the ADI-2 Pro FS R BE is also applicable for the ADI-2/4 Pro SE.

With the ADI-2/4 Pro SE, you get additionally these features:
- more recent ESS DAC with even little lower converter latency
- 5 reference levels in combination with the feature auto ref level a broader range where SNR/Dynamic is being optimized
- if I remember exactly, more D/A filter
- an additional play out path for e.g. a mastering chain
- possibility to connect a turntable, usually MM but with additional preamp also MC cartridges
- new Pentaconn plug for connecting balanced headphones with this new plug type which became popular for this purpose
- a "trig out" port to trigger a device like e.g. power amp to turn on/off (if supported by such a device)

I am using two ADI-2 Pro FS in combination with UFX III
- in the recording corner
- in the HiFi corner in front of a High-End HiFi to perform D/A conversion for all devices with digital ports (TV, Blu-ray Player)
and of course to use the PC with MusicBee as player. MusicBee can nicely be remotely controlled by any Android device with a little plugin called MusicBee remote. All in all, nice synergy of devices for allowing plenty of use cases, be it for recording or HiFi or to use the HiFi as "Monitor B" in TotalMix FX.

My setup just as an example for you here:

It doesn't matter whether you use optical SPDIF or AES, currently I use optical SPDIF connections to both ADI-2 Pro because the cable is thinner. Even a length of up to 16 m (15m + 1m cables and an Oehlbach 4:1 Mk II TOSLINK Switcher in between) still works fine up to 192 kHz (see "+" below).

A TOSLINK cable simply fits better in a small plastic cable duct that is glued to the tile end strip. Easy to open and close when required. It is self-adhesive, but I recommend using Tesa Powerbond (or similar product) at the ends and in the middle, as this is simply more stable.

(+) Please note, according to the standard TOSLINK cable may only be 10 m, but the RME TOSLINKs seem to be selected or simply of good quality and support even longer distances.

Wondering what is so difficult to understand.
Those special features are explained in the blog, also on the product page and in the manual.

The 1st model ADI-2 Pro was intended primarily for studio use.

But also HiFi enthusiasts discovered the many useful features to use it also in front of a HiFi.
Either in front of a preamp or even completely replacing a preamp and directly being connected to the power amp.

It is a reference converter with more/advanced features that a recording interface does not have.
Depending on the model, it is either a D/A or an AD/DA converter.

These devices have many (!) features and use cases.
They can also be used as a USB DAC for playing music, also supports DSD, but not only that.
Or you connect it standalone to your recording interface to enhance the monitoring.

It also has powerful headphone outputs to drive even insensible headphones with high impedance, and some models even support balanced phones or IEM.

Dynamic Loudness is extremely useful if you need to listen silent, then it dynamically boosts Bass and Treble so that the tone does not become too quickly too thin. It is not the static Loudness Switch which is not parametrizable and boosts to much, which is not suitable for every volume. And so on and so on, please do me the favour and read the manual on your own. The first pages already list the enormous features of this device.

[and much more, I can't repeat this all again and again .. please check webpage, manual, blog article, I explained already a lot ]

If you (think) you do not need this all, well then simply skip … it was meant as an offer to eventually improve your setup and to give you a use case how you can make use of the AES port.

You are mixing the products. There is no IFX II, but an UFX II, but you said that you have a UCX II.

The UCX II has one ADAT port, so you can connect either a preamp or an AD/DA converter with 8ch.
ADAT supports 8ch at single speed (44.1/48 kHz).

AES + SPDIF: 2ch @44.1-192 kHz
ADAT: 8ch @44.1/48, 4ch @88.2/96, 2ch @176.4/192 kHz sample rate

You can add an 8-port ADDA converter to the ADAT port and record at single speed (44.1/48 kHz).
With higher sample rates, you loose channels due to port multiplexing (to achieve the needed higher bandwidth).

You need to look which device supports you best for that purpose, the cheapest product is from Behringer ADA8200.
Better multiport AD/DA converter cost more.