A screenshot of your VST CONNECTIONS > EXTERNAL INSTRUMENT screen would be helpful. Particularly how you linked it to the Rack XS.
Also a screenshot of the Motif-Rack XS Editor VST's FILE > VSTi SETUP screen. (Are you using it? Or are you just using an XML voice list?)

we are looking for how you are set to RETURN the audio generated by the Rack XS. You need to select the ASIO driver for your audio interface.

Let us know.

Hi Bad Mister,
I have attached 6 screenshots of my setup and connections.
Please let me know if there is anything else you need.
Thanks for looking at this for me.
ciao, Dan

MIDI Device Manager

MIDI Port Setup

Motif-Rack XS VST

Rack XS VST Setup

Rack XS VSTi Setup

Sorry I could only load one image per reply.
ciao, Dan

Found the solution to my problem from here (probably your work Bad Mister so thanks):

Motif-Rack XS USB setup Motif-Rack XS Editor VST

In the VST Editor setup, the USB DETAIL was not set to ON.

Then in the VSTi setup I set the Audio Return ports for Rack XS 1 to Analog 3 (L) and Analog 4 (R).

That's it!
🙂

In your MIDI PORT SETUP:
In the column labeled "In 'All MIDI Inputs'", you want to mark only the devices that you're using as MIDI controller sources for your MIDI Tracks. Since you have the two MIDI ports of your fireface marked as well as port 1 (Rack XS) and port 3 (presumably your keyboard is connected via a 5-pin MIDI cable to the back panel of the Rack XS and you have two other controllers attached to your audio interface).

In your screenshot MOTIF-RACK XS VST:
You should be using the Motif-Rack XS in MULTI mode so it is 16 Part Multi-timbral... You show it in Voice mode (?)

I've attached a screenshot of my MIDI/Audio devices setup.
My Triton Studio and Hammond XK-C3 are attached to my Fireface 400 via 5-Pin MIDI, which in turn is connected to my DAW via IEEE13944 Firewire.
While my XS-Rack is connect to my DAW via USB.
On my current configuration, I have FF400 MIDI Port 1 connected to my TS and FF400 MIDI Port 2 connected to my XK-C3.

With regards to using Multi mode on the Rack XS, I was under the impression, perhaps falsely so, that in order to use MULTI mode (16 Part Multi-timbral.) I needed to have a mLAN16E2 board installed on the Rack XS.
If I do not need this piece of hardware in order to use 16 Part Multi-timbral mode, please advise on what I need to do in order to be able to use it.
thanks,
ciao, Dan

You do not need any optional board to use the Motif-Rack XS as a 16 PART multi-timbral tone generator. The only way you can have the Motif-Rack XS play multiple MIDI Part simultaneously is to use Multi mode.

You need an mLAN16E2 or a FW16E (option FireWire expansion board) if you want to take advantage of the 14 additional assignable audio outputs.

If you do not have the expansion board, the Motif-Rack XS sends the audio for the sixteen Parts to the main Left/Right outputs. You can pan each PART anywhere in the stereo field.

To use Multi mode:
Press [MULTI]
Press [EDIT]
You can select COMMON parameters or PART 1-16 parameters.

Initially PART 1 is assigned to MIDI Channel 1, PART 2 to MIDI channel 2 and so on through to PART 16 on Channel 16. You can choose a different VOICE for each PART.