Molten lead energy amplifiers present very interesting safety features to exploit nuclear fission in a subcritical assembly driven by a neutron spallation source. To characterize those features, reactivity effects due to geometric, material, and spectral changes are analyzed. A main objective of this study is to find out if reactor subcriticality is kept even in the case of accidents producing large reactor distortions. It is found that this is possible in compact fuel arrays that have a high enough operational keff to yield a huge energy amplification, but the negative reactivity safety margin must be accurately assessed in any subcritical reactor design, as an essential point of its safety report. Some hints for future studies and better nuclear data calibration are also identified.