In making the case that Jesus was the one who preached to the dead the very first issue that we have to deal with is: what does dead mean? How can someone who's dead listen to preaching?
He Also Descended
Who descended and what are the lower parts of the Earth? Fortunately this isn't a mystery and we've already done the work needed to resolve both issues.
Jesus descended into the lower parts of the Earth. We can resolve this as being a place in the lower mantle, we euphemistically call it Abraham's bosom, the same location that he described in Luke 16.