The audio interface in this case is a first-generation PreSonus Firebox unit, which still works faithfully with both macOS High Sierra and Kyma 7.23 on an individual basis. I have four other audio interfaces, each of which are no longer supported by macOS (an Apogee Duet, NI Kore, NI Audio DJ 8, NI Guitar Rig). At one point I remember reading here that the Apogee Duet could still be used by a Pacarana connected to a post-OSX macOS system, as long as one wasn't dependent on having use of the accompanying software utility to adjust signal levels, activate phantom power, etc. I'm not sure if this is still the case, but if the original Apogee Duet FW model can still be used with a Pacarana and Kyma 7.23, then this would resolve my present issue for the time being.
If not then I'd like to obtain another identical Firebox model on eBay since it's so dependable, and run the pair together on the same FW network, with my Pacarana claiming one, and my macOS system the other. I noticed that when I currently use both of the FW ports on the Pacarana, the Firebox connected directly to one port automatically appears available for use in the Sound Preferences Panel of my macOS, which is connected to the other FW port on the Pacarana. This is convenient for when I might need to quickly use the Firebox as the macOS audio interface without needing to physically reconnect anything. My only concern is that if I do acquire another identical Firebox model, might there be any challenging issues that anyone here might already be aware of. I'm mostly interested in an identical model to make use of the S/PDIF in/out on each to achieve a bidirectional stereo digital connection between the mac and Kyma in a most simple manner.