Thank you Enrique; I followed your advice and set the RATE CNV to the max 9000, and it works perfectly.
It also seems to load as fast as it did on the default setting (I forgot to write down whatever that was, 20 something ms?), so that's another cool thing!
Nebula is so remarkable that I want to keep it my own production secret. But then I told my partner to buy it for his production rig (PC/Cubase), and now the cat's out of the bag. Soon everyone's going to have it.
The software's brilliant. I've had it about a week; I've already used it on two TV ad tracks, and took several plugins that I was using before off my machine since I know I'll never use them again now that I have Nebula.