I unboxed the mat as instructed and started to preheat it (great that there is no smell). But after 2 hours at 70°C, the surface of the mat is only 100-120°F. Is my controller defective?

The mat should not be hot to touch unless you cover it with a thick blanket. Amethysts radiate heat, and the surface remains warm — it is normal. When you lay on the mat, your body’s cells will absorb all the heating energy. 

When you become more experienced with the mat, you will notice a big difference between temperatures!