I'm still on the original 1.6 that came with it but if you haven't seen it, check out Rip Tripper's youtube video where he rebuilds one. I've been meaning to give it a try but I haven't gotten around to it. After going RBA/RDA, the nautilus flavor is just too muted imo.
I have seen chatter about the new nautilus mini heads that are coming out (if they're not out already) that are supposed to be backwards compatible and much much better. I may pick up a few and give em a go one of these days.
Sorry, I know I didn't answer your question in the slightest but just wanted to share just in case you wanted to know.
Vape On.