Have you tried using the magic wand? You can select the black and then use Refine Edges in the Select menu to sharpen up your stipples (or do the inverse with the wand, ie select the white areas and do the same). Then fill the selected area with black , so that you have a "pure" photoshop version of your scan. I've found this works quite well. I've never thought to use Curves, though I have used levels, like Geoff. If most of your scan is clean and you just have a few muddy areas you can select those and work the magic wand/refine edges on just that area.