EDIT1: some additions, pls re-read
In regards to your questions:
> When we connect two Micstasies and a single XTC in a MADI chain, what does the XTC's MIDI ID mean
> and how should it be set?
Each device needs to get a unique MIDI ID configured, in your case on the same MADI bus (1..8).
This ID is required to tell your remote control application (either MIDI remote or AUX device) which device to control.
> Is it completely separate from the delay compensation ID?
Yes, delay compensation is a different topic as explained already in this thread.
> And is there an equivalent setting on the Micstasy?
Yes, see Micstasy handbook chapter 11.2 - 11.4: http://www.rme-audio.de/download/micstasy_e.pdf
> I'm also unsure how the MIDI outputs on the preamps relate to the MIDI ports on the MADIface Pro?
Similar to the UFX+, perhaps have a look to this handbook at chapter 25.8.2: http://www.rme-audio.de/download/fface_ufxplus_e.pdf
UFX+ Midi Port 1 = the 1st MIDI port on the UFX+
UFX+ Midi Port 2 = the 2nd MIDI port on the UFX+
UFX+ Midi Port 3 = MIDI over MADI optical
UFX+ Midi Port 4 = MIDI over MADI coaxial
For your MADIface Pro there will be fewer options, but the logic will be the same.
> Finally, when remote controlling the preamps with MIDI Remote do these settings (e.g. device ID)
> override any changes made on the units themselves?
No, its simply tells the application which device to remote control.
So 1st you need to assign the devices MIDI IDs which are uniq for each device.
Maybe you want to number top/down in the rack (1..8).
Then you configure the proper MIDI ID in either MIDI remote or in TM Settings for AUX device.
In your case I would propse that you use MIDI remote for all devices, because there seems to be
smth like a "race condition" currently for all devices when using MIDI remote and AUX device
at the same time and as you have more Micstasy's (2) than XTC's (1).
You can nicely store workspace settings for MIDI remote, that all three windows open at their fix location.
Some general information
MIDI remote and AUX device (TM FX) are two different RME remote control solutions.
AUX device is only available for the Octamic XTC and it controls the most important parameters of an audio channel (like 48V, PAD, Gain, Autoset, ...). One big advantage is the integration into TM FX, that device settings can be stored digitally into TM FX snapshots and thus can be recalled easily (also by a "workspace quick select" operation).
MIDI remote has the advantage, that it is available for all RME devices that are remote controllable by MIDI and that all device settings can be remote controlled. In the case of the XTC things like routing. Midi Remote has its own possibilities to store device setup and workspace.
For the Octamic XTC you need to decide whats more important for you
a) integration into TM FX, to be able to store most important channel settings into a snapshot or
b) to be able to control more/all parameters via MIDI remote
For all other devices you do not need to decide anything, as the XTC is the only device which supports AUX device.
For one device - the XTC - you can not use/enable the 2 different remote control solutions at the same time
AND for the same device, as they would influence each other.
Whats should be possible is, to use for the XTCs AUX device and for other devices MIDI remote, but there are currently side effects that these two solution influence each other on one MADI bus.
If you have a recording interface with different MADI busses it might help
a) to distribute the devices differently on the MADI bus so that only one remote control solution is active on the same bus or
b) to use only MIDI remote
X10SRi-F, Win7 Prof, Cubase 9.5, UFX+, Octamic XTC, ADI-2 Pro FS BE/DAC, RayDAT, ARC USB