"The optical brighteners that give MULTIGRADE papers their brilliant, sparkling whites stay exactly where they are needed - in the highlight areas of the print. All MULTIGRADE papers, except MULTIGRADE FG WARMTONE, have anchored optical brighteners which means they won't wash out and the paper stays white. With MULTIGRADE FB WARMTONE, the brighteners can be removed with extended washing for an even warmer base tint."
I've also posted this info on the LF forum some time ago.
This is absolutely noticeable as well. When I want extra warmth I wash for about 1.5 hours. It's something you want to test because you need to account for it with your drydown when printing.