Hi Andoran, welcome to the RME forum.

You are actually planning a very nice setup. UFX * and Octamic XTC is a very pleasant setup.

A MADI-based setup I can recommend much as it gives you the most flexibility in terms of placement of devices (cable length) and also some spare channels for later expansions.
Moreover, useful if you need some spare channels for loopback recording if you want to record special submixes, then you do not sacrifice other needed analog or digital channels like ADAT for that purpose. Usually, they are already being in use for other purposes.

The Remote control feature that you are referring to is called "Auxdevice" support.

Auxdevice is currently only supported by the Octamic XTC. It makes it possible to remote control the most important parameters of the Octamic XTC by using TotalMix FX.

The good thing is, that all these preamp settings can be stored in the TotalMix FX Snapshots and Workspaces and you have the control in one TM FX window.

For Auxdevice to work, you require a MIDI connection between the recording interface and the Octamic XTC.

If you have an RME recording interface which supports MADI, then you can use the so-called "MIDI over MADI" feature and do not need the additional MIDI cable (which also has length restrictions of approx 5 m). Then the remote control happens through the MADI cable along with the audio data. RME uses unused bits in the MADI protocol header to perform the MIDI communication with one or more devices.

TotalMix FX is, for my personal taste, the best remote control possibility because you can do all the relevant stuff inside TotalMix FX.

Incidentally, if you would purchase an RME UFX III, then you would have all the benefits of the new interface. Numerous things changed between the old UFX and the UFX III, see my comparison Excel, here where you can find it: https://forum.rme-audio.de/viewtopic.php?id=35156

In this case, (UFX III) you could connect the Octamic XTC through MADI and use Auxdevice and MIDI over MADI. No MIDI cabling required.

The UFX you can connect through ADAT to the UFX III. For the management of the unit, I would keep it connected via USB so that you can operate the UFX using a second TotalMix FX instance. As an option, you can still/additionally operate it through display if needed.

PC
| |
USB
| |
| +-- UFX III (controlled by TM FX instance #1), clock master
|        | |   | |
|        | |   | | [MADI]
|        | |   | +Octamic XTC (gets clock through MADI)
|        | |   ++
|        | |
|      ADAT1+
|      ADAT2
|        | |
+---- UFX (controlled by TM FX instance #2), gets clock through ADAT

Note: for recording interfaces like the UFX there is no other control than either by using TM FX or to operate the unit in stand-alone mode. But I would recommend keeping the UFX connected to USB and operating it through the 2nd TotalMix FX instance. You can make the window smaller and limit it to those up to 16 channels that you need and which you route through the one or two ADAT ports to the UFX III, as shown above in the ASCII drawing.

Regarding clock synchronization: in the setup above, the UFX III would be the clock master and the other devices would get the clock through digital inputs (ADAT, MADI). No need for Word Clock cabling.

Side note: there are some other remote control solutions available from RME in historical order
- MIDI Remote (for all devices using MIDI or MIDI over MADI)
- MIDI Remote AVB (for 12Mic, AVB Tool and Octamic XTC using MIDI or MIDI over MADI)
- RME Connector, the successor of MIDI Remote AVB which adds remote control through network (LAN, WLAN, …)

Hint: if you use TotalMix FX Auxdevice to remote control your Octamic XTC connected to MADI then do not use another Remote Control Software in addition to TM FX Auxdevice. Because then the Octamic XTC gets confused, and random operations are being performed on the device, which is not what you want.

But as the remote control via TotalMix FX and Auxdevice feature is anyway the best form of integrating the external preamp into TM FX and to remote control it, you wouldn't want anyway any of the others on top. If you have everything in TM FX, then you also do not need to reserve space for another window on the screen.

You do not even need to open a 2nd TM FX window, you can select which interface you want to show, the selector box is in TM FX on the top right.