Nicholas, a few others have aired the same complaints on this forum, and it mystifies me, because PMK is the best developer I've ever used by a large margin. The only straw I can grasp at regarding paper is that both Ilford Multigrade IV and Kodak Polymax II have slightly lower highlight contrast than some other papers like Agfa Multicontrast Premium. If you're using one of the former, you might want to try the latter, but I doubt you'll find THAT much difference.(posted 8415 days ago)Otherwise, I have to wonder if your exposure and processing need fine- tuning. It's been my experience that I pretty much need to cut the film speed in half with PMK in order to get a rich, easily printable negative. If that sounds like what you're getting, then you might try increasing your developing time to get more contrast. It should not be necessary to resort to amidol for adequate contrast.
But, to answer some of your questions, Gordon Hutchings recommends adding between .05 and .1 grams of amidol to a liter of PMK in order to increase film speed by 1/3 to 1/2 a stop. He says .05 is sufficient and that more than .1 grams will increase fog. I've never added amidol so I don't know if there's a color change.
You don't mention what film you're using, but Ilford FP4+ gives nice results rated at 64 to 80. When exposure and development are right, the low-contrast effect you mention works mainly on highlights that otherwise would be unprintable which, of course, is a good thing. Good luck dialing it in.