Thank you for your message.
I do not see a problem to record your hardware synth to an audio track in Cubasis.
However, recordings via the iRig seems to be limited to mono tracks, given the number of hardware inputs of the device.
Have a look at this tutorial, to learn more about how to setup Cubasis for audio recordings.
While in this example, a guitar track is recorded in Cubasis, the steps are somewhat comparable to your use case.
To use AmpliTube with Cubasis, the app will have to be loaded either via Inter-App Audio or Audiobus,
To my knowledge, AmpliTube does not support the Audio Unit format.
Hope that helps.