Thanks for trying and confirming @ggmanestraki . You’ve helped me enormously and it feels like we’ve got SO CLOSE, but… that’s a bit of a showstopper - saving a preset but having parameters revert to zero every time you tweak them is definitely not optimal.
I’ll see if I can drum up any answers on the Helix Native forum. And in the meantime, if this hasn’t put you off, the special offer on Native (and other Helix software) expires on March 31st, so if you’re still tempted, it’s a good time to buy 
Thanks again for all of your help, you’re a star.