I would suggest that you put after the softener. Softeners add Na to the water that may effect the solubility of your sea salt. Plus some softeners add chemicals along with the salt to soften the water. The RO/DI will remove some or most of the above plus any other dissolved nutrients. I wouldn't worry about the removal of Ca,due to the fact that you will add seasalt(which has Ca in it) and/or Kalkwasser.