Lack of light or Cal-mag?

Plants look healthy, upper leaves have no symptons, so if it were a cal/mag problem, all the leaves would be showing signs, not just the bottom ones, therefore your thought that it is a lack of light, would also be my diagnosis. Remember too, more leaves = more energy production = more growth, so if you feel a need for defoliation, go easy on it, probably best to leave all healthy green leaves and only remove yellow ones. Personally, I am very anti defoliation though. The "lollipopping" you have done is what I consider ideal, a really nice job indeed!