Manfred,
Thanks. Good idea.
The first test print is on Canson Baryta, but I have a stock of Rag Photographique, so I could try both.
I have started putting up 8 x 10 prints in my study, using standard mats to make it easy to swap them out. It's a cheap way to have time to look at test prints, and it livens the place up. I'm increasingly inclined to follow your suggestion and darken the tiles a bit.
Dan