I know these white specks from a long time back, looked at them under a 10x magnifyer and microscope, and as far as I know, they are a problem related to the sizing of the paper and very difficult to come by. I don't think changing from rod to brush helps. Sometimes you think you have the solution, only to see them re-occuring in the next print. If I remember correctly, I had them on Watson Watercolour and Rives BFK. Aches Platine is notoriously changing, I would not advise it. The news on Artistico, however, distress me, as this paper has, as far as I know, always been pretty consistent.
My advice is: change the paper, to Artistico, I would have said, but...
Expensive but a sure bet is Buxton paper. And what about this new American paper whose name I have forgotten? There was some news it had been changed and newly marketed???