UPDATE:
I exported a clip from resolve and found it looked different in QT and in FCPX, but not in VLC (where it was identical to Resolve preview)
I brought the clip in to FCPX and exported it as and MP4 and a MOV, both of which, again, looked exactly like the resolve preview but only when watched through VLC.
Conclusion being, I’m going to trust the resolve preview, ignore discrepancies in FCPX and Quicktime viewers, and hope that the flattening/desaturation (gamma?) doesn’t occur when viewing on the web/HDTV etc.