Thanks, Anand and Young-Ho, for information about FSK in traps. I agree, it's important not to deaden the room while absorbing bass. I have experienced that (after using too many broadband absorbers), and it's not a great result.
Many commercial acoustic panels take a combination approach, covering all or part the rockwool or fiberglass with with perforated or slatted plates, strips of mylar, or membranes that diffuse or reflect higher frequencies. It's good to see homebrew options for those rolling their own.