Even is far distant for the opening of this threat, I thoaght to wip the dust maybe for other who still have the same problem.
Same problem with OKI c 9600 & ES 3640 a3, i think is a matter of RIP (flaky emulated Postscript) combined with narrow color gammut ICC (i have seen it in while soft proofing in corel or Illu, or Phot in the Fill Tab color box were certain areas of dark Red, dark Green, dark Blue are substituted with gray, don't know why? Also i think it also a mater of agresive GCR color replacement instruction. you can prevent certain colors for not printing gray but you can not control photographic gradients RGB or CMYK workflow, or the middle areas of two point gradient Red-Black( siple or comp) Green-Black( siple or comp) Blue-Black( siple or comp)CMYK environment.