Thanks, Mandycat and Catbird,
Chami is the "choosy" one of the bunch, and is the most sensitive about cleanliness. Even when she eats, if a piece of kibble falls out of the bowl while she's eating, she'll go around the bowl to eat that piece before going back in for more.
One time when they had just started living with me and I wasn't sure how the silica worked, I went to the box to stir it to even out the urine. Chami came to watch. I thought about changing the crystals, but then wondered maybe there was another day left in them, so I stirred the crystals and started to leave. Chami looked up at me, meowed a couple times, and then when she saw that I was looking at her, she started urinating outside of the box!
I immediately changed the litter and cleaned the box, and there was no trouble after that.
I thought she was just brilliant!

(The other 3 don't seem to mind if the crystals are a bit yellow, but Chami definitely does not approve.)
After Chami finishes her business, she tends to run away from the box without looking back. The other 3 perch to clean (where Chami perches to do her thing), and only Yoyo and Lulu sometimes cover theirs (I'm fairly sure Mooch is the boss of the group).
I guess since Chami seems to be using the new box and litter without a problem, we'll stick with it and see how it goes.
Thanks!
LOL about the scratching metal! Ikes!