Hi Blighty,
For me, Multigrade IV FB starts to "eggplant" in the darker print values at 1:9 with Kodak Rapid Selenium Toner (KRST) for 3 minutes. KRST at 1:10 for 2 to 3 minutes works for me. There is a definite darkening of the darker tones. I would expect colour change would have alot to do with your choice of developer.
To be sure, make 2 max black patches & tone 1. Make 2 max white patches & tone 1. Compare. Try this at several dilutions. Take your test negative (the one that prints the easiest with the least work at normal contrast & development that has a full range of textured print values, and a black) and print off a bunch. Leave one untoned and try the rest at different dilutions and times.
Nobody but you will know what is right for your images.
Murray