If it looks like concrete but sounds hollow it is probably gyp-crete over a wood floor. Gyp-crete is a thin cement applied for leveling purposes, it doesn't affect the strength of a floor significantly.
In my experience your floor will be fine with a 125G going either with or across the joists. As long as you have a good solid stand you are going to be okay.
In my experience your floor will be fine with a 125G going either with or across the joists. As long as you have a good solid stand you are going to be okay.