Well, let me answer my own question: keep trying. It's the steep learning curve. Studio One has plenty of hidden features and not always everything works as should. Not all sets of instruments worked on the first try, for example -- only the first I used produced sound, then one by one they came up to life. I also had to make sure I deleted all the external instruments I had created while trying to make it work, some set to use USB96, some not. I also disconnected and swapped the midi cables, just in case they were in the wrong place. Now that midi works, I'm stuck trying to make a USB microphone (T669 from Fifine) work with Studio One. It works with the Mac, it was immediately auto-detected, it's set up to use it, but at this point it's not an option in Studio One, only the left and right default options show there. More to come on this...