[Righty27]

... often not appearing as available on my Mac e.g. system preferences, even after rebooting both iMac and Eleven Rack - no hub.

I wonder if the Avid Duet is actually identical to the Apogee version? Does it work with Core Audio (with Pro Tools) on a Mac or dedicated driver, like Eleven Rack?

When yours loses connectivity, does this affect just Pro Tools or does it continue to work e.g. with iTunes?

If Apogee USB interfaces that use Core Audio continue to work after OSX upgrades without firmware updates, I guess the Avid model should work even if no longer supported by Avid in future ... and if it is identical, there should be no reason for this to suffer from connectivity issues any more than the standard Apogee Duet 2?