– However, if I am using the motif as my audio interface then I cannot have a second audio interface for the computer.

– Also, am I correct that an audio interface is not only involved in bringing sound into the computer, but also involved in playing audio back, so any play back – a Cubase project, youTube, soundCloud etc. – will go through my motif.

The details here depend on whether your new computer will be running macOS or Windows. Each OS handles things differently and comes with its own unique strengths and weaknesses in working with your Motif XF.

If it's a Mac, first of all the Yamaha Steinberg Firewire Driver (aka YSFW) is presently only available as 32bit software. This means that as of macOS 10.15 Catalina (released in Oct 2019), the driver does not work at all because Apple has completely dropped support for 32bit software. Yamaha/Steinberg have not released an official statement, but the word on the grapevine is that they do not plan to update it. Unless they break precedent and release the source code so that the community can cover Yamaha's negligence, this essentially means that they may be choosing to actively kill the driver dead as far as modern Macs are concerned. If you care about this, please vote on IdeaScale.

With that said, it is still possible to use a Mac if you can manage to get it running an older version of macOS. In that case, Mac's strength with regard to the Motif XF and YSFW is that you can create an "aggregate audio device" at the system level. This would allow you to combine both the XF and a second audio interface, which Cubase would then see as a unified single audio interface.

On the Windows side of things, there is no such system level support for aggregate audio devices. But, on the flipside, the YSFW driver for Windows has the unique capability to act as two audio interfaces simultaneously. One runs in ASIO mode, which is the main low-latency mode that you would expect to use inside Cubase. The other is as a 2-input/2-output WDM audio device: this is where general Windows system audio would be directed, such as YouTube, SoundCloud, etc, as well as system audio input (like when you connect a microphone directly to your PC sound card for use in video conferencing). This is unique because ASIO audio interfaces typically instead run in exclusive mode, meaning that when Cubase is using the hardware no other software can access it.

The YSFW Windows driver control panel cleverly allows you to set which of the Motif's outputs are routed as the WDM stereo input (this could be the Motif's stereo output, the A/D Inputs, or any pair of the Firewire digital-only output channels), as well as where WDM output is routed out to on the Motif: out the Main L&R jacks, out the Assignable L&R jacks, or into the XF's digital input channels (that's right, your Motif has two more channels of audio input besides the A/D input). I've used ASIO and WDM in tandem in order to do many interesting things, such as loopback recording of system audio (games, videos), routing videos of drummers on Youtube into the Motif's vocoder for percussive synth jamming which can be simultaneously recorded into Cubase, as well as all-in-one broadcasting of Motif+DAW sessions on Twitch. This is actually more flexibility than is available with Yamaha's latest synths (Montage/MODX) using the YSUSB driver...

That's a lot of information, but I hope it helps.

Very helpful. Thanks.
I'm planning to get a Windows desktop. From what you're saying, it sounds like this combination offers a lot of flexibility.
How would you rate the motif as an audio interface in terms of audio quality? I mean, to my ear, just based on recording in sample mode with a dynamic microphone, it sounds very good. But I don't have much to compare it to.