I would find a natural background like a wooden cutting board, or a nice piece of slate for the knives. Light the scene from above with your softbox for a nice even, virtually shadowless light. You may get a nice catch light from the softbox in the shiny blade of the knife, depending on how you angle it. You definitely want the blade to appear very shiny. This is the selling point. It will make it seem sharp, and high quality. If need be, to get more highlight on the blade, you can use the small flash directionally on the blade for added punch. I would angle it very obliquely to the blade to draw out shape and form.
I would use the long end of your lens, (45mm) and try and stop down to around f/8 or f/11, to get a nice sharp image. You can try isolating parts of the knife with closeups to show the detail. For these you might require a set of extension tubes to bring the minimum focus distance of the lens closer. A closeup filter is also an option.