Speaker refinish

Even if Steveaustin's ProAcs weren't an oil or wax finish, an oil would have penetrated through the sides of the cracked veneer. Veneers with tiny little knots (like some Yews) or burls have a distinct tendency to craze & crack over time. Once the surface is broken with a crack, then oils can get in and do their thing. As Lars says, things like Scots do make wood look pretty. But, it's just a surface film that doesn't penetrate most wood finishes. Furniture polish manufacturers tend to suggest their products can accomplish things that are unlikely, such as nourishing wood. Rather like the implied promises of women's cosmetic moisturizing products :-)

I had a pair of Proac Response 1.5s in cherry finish with same problem. I left the grills off the entire time I owned them and never noticed any improvement. When I sold them the difference in the finish was noticeable in the pics.

Proac uses plastic coat. What you have is sun shadow so exposing area under grille to UV will cause finish to match up shouldn't take more than a few months.

I can wait a few months. I may just put them in direct sunlight not too far from the window while I am away on holidays this summer to speed things up.

Many things can darken or lighten a finish. In this case it is most likely UV, but could be a reaction to the grill cloth and/or the grill frame from the off gassing of the finish. Common catalyzed lacquers (the most popular finish) can take over a month to cross link and fully cure. Prior to that formaldehyde amongst other things are emitted that reacts to the finish or veneer. I'd try putting it in direct light for a few weeks. A lot depends upon how much UV blocker is added to the finish by the finish manufacturer.

JohnK,

I know you are a fan of shellac and the process called french polishing, however, shellac is a plastic too. Furthermore, anything beyond the first coat of finish or common penetrating stain is all of the penetration you will ever get. Once it is sealed, you maybe able to melt into the finish, but not through it.

Wiki:
Shellac is a natural polymer and is chemically similar to synthetic polymers, and thus can be considered a natural form of plastic. It can be turned into a moulding compound when mixed with wood flour and moulded under heat and pressure methods, so it can also be classified as thermoplastic.