I was told the recommended way is the method Doug from The Sound Test Room demonstrates:
This means you have two tracks per instrument so to speak, a MIDI and Audio one. Personally I think this is cumbersome too.
I suppose in case with the MX88 (I own a MODX myself) you need to set up multiple of those midi tracks for one summed return audio track. Not sure if the MX88 can feed back multiple audio channels over USB into Cubasis, probably not.
You can also use AUM in the background with IAA connections to Cubasis, but I found that to be unstable. Jakob Haq demonstrates this method here: Record from AUM into Cubasis 2 & 3 | IAA Output Routing | haQ attaQ - YouTube
I’ve posted a similar question here recently: Using hardware synthesizers in Cubasis
I wish Cubasis would offer the same possibilities here as Cubase, but this isn’t the case yet, and it might not be for a while. So the workarounds are currently the only options.
Fijne dag verder! 