A speaker of that calibre I would seek out a professional to refinish them. I would remove the drivers etc. before taking them to someone. If attempting it yourself, you will have to completely strip the outer poly/sealant coatings off which will be labor intensive and time consuming and very messy (done it many times myself but not with a high end expensive speaker like yours). Since you want black, you will not have to remove the stain on there now, but I would consult with a furniture repair shop on what they would recommend you use. Home Depot has a stain section and they probably have something like a black oak or very dark stain. Hope I had something to contribute here and not just everything you already knew.
