Hi Mark,
Thanks for the excellent question, and thanks for "getting" that the more info provided the better, and therefore more useful, the answer.

You are using Cubase Pro 9 on a Mac, this gives you many different workflows. Which one or which ones you choose will depend on what exactly you need to accomplish. We will give you several different things that will hopefully help you decide.

Personally, I use many different methods simply because I have to in order to help others, so I reconfigure my setup almost on a daily basis. I have a setup for using the Montage alone- as you know it has its own built-in audio and MIDI interface that makes it self-sufficient when it comes to recording with a computer. You do get the advantage of 32 audio bus outputs. If you are doing that type of Project where you need separate audio outputs in your tracks, nothing beats this flexibility.
I have a setup where I'm using an external audio interface... I have yet another where I have several audio interfaces simultaneously...

Routing Parts or even individual drums to audio outputs is fairly straight ahead once you get into it.... just as being able to configure and reconfigure your setup. You have an audio interface that you are using for your other synths... you have discovered that Cubase certainly allows for multiple configurations that can be Preset and recalled at a click, so like any modern day recording studio that rents time for hire, you can have several different setups stored and ready to recall. Excellent!

You should definitely get Montage Connect into your workflow - it will Or it can automatically store and recall the Performance you use for the current Project. When launched with your Project, it can work to capture every setting and it bundles it with the Cubase Project file... all you need do is open your Project and Montage Connect can automatically bulk dump the Montage Performance back to your instrument.

This means you do not have to store the Performance in your Montage User Bank (of course, you can if you wish) the data is bulked back to the current Edit Buffer of the Montage. This means even if you don't remember every sound you used - your Cubase Project restores everything automatically (via Montage Connect). No hunting for a Performance, no necessity to find a file and load it- it makes getting back the exact data you were working an absolute no brainer.

I am currently working on a tutorial that introduces Cubase Pro 9 users to the "Montage multi channel record" template.
Go to FILE > NEW PROJECT > select "Recording" > select "Montage Multi Channel Record"

This setup template creates 16 MIDI Tracks for the 16 synth Parts, 16 Audio Tracks for the 16 available stereo buses, it sets up Montage Connect to capture/backup/restore your Montage Performance for *this* Project. Additionally, it sets up a MIDI Track for System Exclusive messages which can be used to capture "global" commands (specifically: Super Knob and Scene changes).

With this template you can record MIDI, you can record Arpeggios, you can record audio, you can record a combination of tracks - some MIDI, some audio. What you want to learn is how to do (assign) what you require. You'll want to understand how to use this flexible tool. This template makes use of Cubase Pro's INPUT TRANSFORMER - a fancy configurable filter.

In Cubase Pro, the INPUT TRANSFORMER is used to create a situation where each MIDI Track can only record the data as specified: Sysex Track has the Input Transformer that Filters all but System Exclusive messages. Each of the numbered MIDI Tracks is set to reject all but a correspondingly numbered single MIDI channel and is set to reject Sysex. You wind up with a record setup that gives flexibility and advantages, including being able to isolate your Super Knob movements and Scene changes on their own Track (if ever you need to isolate a Part and just its response to a Scene change or to Super Knob movement you will really appreciate this).

Recording with Montage is going to involve deciding if you need to record MIDI, and if you do, how to go about rending the MIDI data to audio. If you think about MIDI recording as your "work in progress" then audio recording is a commitment to a more permanent form. While data is MIDI you can notate, edit, rearrange, change your mind about the instrument, etc., audio rendering is your first commitment to a permanent form. You can record your MIDI data, edit it, and keep it in your MIDI folder, muted (the ultimate "undo"). The rendered audio becomes your finished item.

Because the Montage can be used as a 16-Part multi-timbal tone engine, or it can be used as a powerful single entity synth... means that at some point getting good at rendering audio will be required.

Sure you could use the Montage, like the Motif series, where each of the 16 Parts is on a separate MIDI channel... but you can also approach the instrument as if it was just a single very powerful synth... if I'm doing a Project where I am featuring the acoustic piano sound, "CFX Concert" with its 10 velocity layers and Key-Off sound, I would setup to use the Montage as a single powerhouse synth... I would render my backing tracks as audio so that I could then use the full power of the Montage engine to recreate my acoustic piano sound.

Rending of audio is quick and easy in the Cubase Pro 9 version (this will be covered in the tutorial)... and you will want to be adept at doing this because it is an essential tool in getting the most out of Montage.

You are thinking correctly about being able to switch (swap) audio interfaces when you require or desire... Cubase is able to switch audio interfaces during the session... (very cool feature). Since you have many synths and three audio interfaces... you will need to decide, based on your own preferences, exactly the best way to proceed.

I would base it on the project itself. Of course, you can switch drivers... this can be daunting because the device acting as the audio interface needs to be connected to the computer and to your monitor speakers. Logically, this can be a nightmare if you don't setup and prepare for this contingency. Since you have a Mac, you can setup an Aggregate Audio Device, that includes all of your audio interfaces in one setup. Macintosh computers can be setup so all your audio interfaces can be active at once. Instead of just selecting one driver in DEVICES > DEVICE SETUP > VST AUDIO SYSTEM, you select your Aggregate Audio Device which can handle all the inputs simultaneously.

The device acting as the main audio interface ("main" here refers to the one that connects to the speakers) must be able to allow you to monitor the other devices. What this means is: while the Aggregate Audio setup will let each of these devices to send digital audio into the computer, in order to properly be able to monitor your instruments, directly, there must exist a connection between each device and the main audio interface.

Advantage: once configured you do not have to reconfigure each time
Disadvantage: your overall latency does increase - this is only a bad thing when attempting to record with plugin effects...

But mostly, you will be working away from latency... if you can setup to Monitor Direct.

Swapping Drivers vs. Aggregate Audio
I would, if I were you, experiment... find out which method makes you the most comfortable. I can say this, once you get comfortable, setup and reconfiguring is not a hassle.

As to whether I think some folks are missing out on one of Montage's best features, hmmm, I'm a realist about this... I realize most people never finish a project... like Impressionist painters, many musicians never finish a piece of work... they finally just stop working on it. But when you are into finishing your compositions, the audio feature of the Montage is one of the most powerful tools. For both traditional and the yet undiscovered non-traditional uses, Montage has one of the most flexible routing situations ever in a music synth, to be sure.

It is not for everyone- for example, since you have so many synths, you may decide to use each for its charm, for what it does best. You may only need stereo output from the Montage... or stereo plus the two analog Assign Outs... that will be plenty for many. Being realistic, not every project requires 32 bus outputs... sometimes simple is better. But then, there are those Projects, where pulling out all the stops is exactly where you want to go.

So, it depends... good thing to know is, you have choices. And since you are not under time pressure, I recommend: experiment. We can offer our help toward your experimentations... once you discover (one of those non-traditional uses) routing external devices through the Montage Motion Control Engine, you may find you've invented something unique.

Be flexible, learn the tools, then let your creativity steer you in the right direction.

I am currently working on a tutorial that introduces Cubase Pro 9 users to the "Montage multi channel record" template.

I'm looking forward to that!

I spent some time with this template a couple of days ago. It occurred to me that it might be useful for the template to come with a Montage Performance, either as part of the template via Montage Connect or as an X7B file.

We'll show you how to create your own default Performance which Montage Connect will automatically send to your Montage when you launch the Template.