Speaker refinish

I have a pair of ProAcs in the Yew finish. Try simply applying some Pledge (Extra Moisturizing w/lemon oil) - available at any grocery store. Spray it on a clean cloth and then apply (DON'T spray right onto the finish). Apply it liberally, but carefully (careful to ONLY get it on the veneer, not the hardware, any matte finishes, or, especially, the drivers/tweeters themselves). Let it soak in for a week. Reapply. It does a great job of conditioning the veneer (mine was starting to get a little crackly-looking around the knots in the finish) and it will slowly darken the finish slightly, but more uniformly after several applications.
It's inexpensive, and it won't (in my opinion and experience) hurt anything. It's not waxy either, so no discernible buildup.

Just be aware that only oil or wax finished veneer will be affected by what Steveaustin suggests. Most speakers are finished with a catalyzed impermeable finish that no oil or conditioner is ever going to penetrate.

Every year or so I use Scott's Extra Gold Wood Cleaner and Preservative on all my furniture and audio gear wood; never a problem, always an improvement, with regard to luster and color. I'd recommend trying it, being careful not to overly contaminate drivers and other areas not made of wood. The biggest factor, of course, is keeping sunlight consistent on all areas; that will only take time!

So I take it that Proac's "yew" finish is an oil or wax finished veneer ?

Thanks for your recommendations, I appreciate it.

If you leave the grills off, they will darken surprisingly fast. I had the same issue with a pair of Spendor 9/1 speakers. They looked even after about 6 weeks of normal light exposure.