I have no experience with Japanese stones but .... I was on So's site today and read that he recommends using a diamond plate on very hard stones. He says that the nagura if it is softer will mix with the grit of the hone and cause scratches. So if yours is a harder stone a diamond plate might give better results. Below is a quote from So's webpage referring to diamond plates for generating slurry ;

Secret MUST item for using hard natural stones. Nagura stones cannot be used for harder natural stones, nor on various Suita stones. The polishing parcticles of Nagura is larger than the particles of super fine finishing stones, and when the stone is hard, the Nagura's particle will scratch the bevel before they are ground to smaller size. Using diamond stone to create the initial slurry, you are getting the slurry that is made of the finishing stone itself. You'd be surprised how much better result you get by this technique.
Also, when you use a Nagura on Suita stones, the coarse Nagura particles will get stuck in the Su (the holes in Suita stones) and will keep scratching the bevel, because these particles won't get ground to smaller sizes.