Another option to the Meguiar's, and also from the automotive world is 3M Swirl Mark Remover (part no. 39009) This is especially formulated for dark color cars, and was recommended by show car polish manufacturer Sal Zaino when I needed to remove some scratches and water marks from my black Corvette. Sal's suggestion carried a lot of weight with me because he is averse to applying ANY abrasive product to automobiles, and for him to recommend 3M Swirl Mark Remover meant it must be effective and gentle. It worked for me. Use a sponge applicator and go very lightly. Of course, we are all assuming your speakers have a paint finish akin to Merlin loudspeakers, and not gloss laminate. If you have gloss laminate, I'd suggest testing any auto polish on an inconspicuous area...like the bottom of the speakers.