@AadamsWell color me embarrassed. I unhooked that little wire and put it back, tightening everything down really good. Seems to have solved the issue!
One thing I noticed when I replaced the midrange was that it had two rectangular blocks that the wires passed through that the midrange I took out did not have. The picture below is the midrange I removed. The one I put in had these two rectangles on the back of the magnet that the wires went through. Anyone know what those were?
Also - I did throw away all the crumbling rock wool. I've got some actual wool and cotton blend shown in the pictures below. Think this will work to replace it?