The zebrawood does darken quite a bit, even without direct sun. My Super 7 XRS (long gone) I ran with the grills. When getting ready to ship them out, I was a bit concerned when I took the grills off and saw a marked difference between the exposed and unexposed areas. Louis was reassuring that they would even out if I left the grills off for a while and he was right. I can't recall precisely, maybe a couple of months, and they did even out. I guess that points to the majority of the darkening happening fairly early on.
Not a bad thing though. When they darken, they get richer looking. Like there's more depth. Would be disappointing if one bought them to match lighter furniture or other components though.
Zebrawood is my favorite. I'm upgrading to Super Alnico HO Monitors and Zebrawood they will be.
Here's a photo of my current SAMs with darkened zebra wood: .............Peter