I use Resolve Carpet/Fabric cleaner. Remove the grills, spray it on, wait 5 minutes, rub/pad with a damp rag and let air dry. Works perfectly every time.
I used ArmorAll automotive upholstery cleaner on my KLH Model Seventeens with great results. And, they smell real nice, too. I am going to try the "Dry" Carpet granules on my OLAs to see if that works.
I used Tuff Stuff I think a couple weeks ago on some black grills, it worked very well. The tops of these (1978) were very dirty, I first blew them out with an air hose then basically followed the instructions given on the can.