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It's hard to know the finish that's on these speaker with your photo. My opinion is the factory did a simple finish and something they can repeat and anyone can do.
Bleached walnut veneer, no stain and a clear coat of lacquer or a wipe on finish and then they wouldn't even need a spray booth. They might have also used a wipe on stain sealer/finish over the bleached wood and left it at that.
I get the impression that you like the color and all that bothers you is the flat sheen of the finish, you want it more shinny in other words.
Minimal, Wipe the cabinets down with Orange-Glo, I use this all the time, it's a thin oil cleaner that will not build up and evaporates fast.
A bit more work, You can get some Mohawk Pour-N-Wipe, it's a wipe on polly finish that dries in 4 hours.
http://mohawk-finishing.com/catalog_browse.asp?ictNbr=215
This is a finish and will build up the more coats you do, and no it will not look like plastic marine varnish that most people think about when one mentions "Polly"
Personally
I would for starters just wipe them down with Orange-Glo, clearly they wanted a dry natural look when making these speakers, that's the more modern look these days.