possibility of refinishing speaker

Well, for one thing, you better be dead set on keeping them for life. Once you do something like that resale value plummets to a fraction of their previous value.

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.

Cover them with satin black duct tape. At least you could remove it someday if necessary.

I'm almost sure your speakers are finished with a 2 Pack sealer and clear over a spirit stained veneer. After preparation, satin black 2 pack can be sprayed over the original clear coat, a sealer shouldn't be necessary. 2 Pack is nasty and should be applied in a spray booth, so a good well equipped spray painter can do the job. I did a quick search and it looks like your speakers were sold in black also. Popularity of solid or clear finishes may determine resale value. I hope this helps.