This is an interesting one. What is the external subwoofer?
Since the “subwoofers” in the soundbar won’t be nearly as useful in bass as a real subwoofer, and won’t likely have much shared FR I would just blend them in as much as you can to your liking. “0” is probably fine but I would adjust it up and down and listen to some content with good bass and see if the soundbar sounds bloaty or whatnot, then back it off.
AFAIK there’s no way to actually level match this thing, so it will all be by ear.