Ok, so I just did some further troubleshooting and for some odd reason I think it may be macOS related on my end. My main rig is with Pro Tools on High Sierra and none of Christian’s .DS files work with any plugin format. AAX natively, AU/VST/VST3 with a wrapper, or Standalone. All my other Pianobook samples/.DS work fine.
Now, my laptop is running macOS Mojave and Christian’s samples work with DS in Standalone and AU in Logic 10. How strange is that? It seems highly unlikely that only his .DS files would give me trouble in macOS High Sierra, but this is the only thing I can think of.
And as I said above, I can get his .sfz files to work with sforzando in Pro Tools on macOS High Sierra, so there is something about his .DS files and High Sierra that doesn’t jive, or that’s what it appears to be.