problem chaining Presonus Firestudio Projects.

I had one FSP and had to switch off between plugging in my midi device outs and my drum mikes, so I bought a 2nd one. I was having a problem with the 8 mic recording setup where I was getting distortion on all tracks. Presonus said to disconnect the other unit as a first test. That worked and the one problem went away. When I hooked up the other FSP again, I did a test record with the midi devices and there was no distortion.
But now, I don’t know why I can’t hear any of the midi devices anymore. The manual says that the ins of one don’t automatically out to the other and it must be set up in the music software -Cubase.
But I can’t figure out how to assign midi devices whose Cubase outs go to a MOTU Midi Express 128, to audio ports. So though a Proteus PK2 has midi in and out to the MOTU and computer via USB, and the audio outs go to the FSP, where in Cubase would the audio assignment go?