Ilford Rapid Fixer is an acid fixer, and contains sodium sulfite as a preservative. Sodium Sulfite is a salt and will reduce or inhibit, or eliminate stain/tanning. That's one of the reasons why staining developers use minimal or no sulfite preservative.
Originally Posted by Tony-S
So if you're using a Pyro developer and want maximum imagewise stain, you don't want to use chemicals that contain significant amounts of sulfite after development. Use a fixer that doesn't contain sulfite, and don't use a hypo clearing agent in your washing process.