Thermoelectric devices directly convert heat into electrical energy and are highly desired for emerging applications in waste heat recovery. Currently, PbTe based compounds are the leading thermoelectric materials in the intermediate temperature regime (similar to 800 K); however, integration into commercial devices has been limited. This is largely because the performance of PbTe, which is maximized similar to 900 K, is too low over the temperatures of interest for most potential commercial applications (generally under 600 K). Improving the low temperature performance of PbTe based materials is therefore critical to achieve usage outside of existing niche applications. Here, we provide an in-depth study of the cubic NaPbmSbTem+2 system of compounds (m = 1-20) and report that it is an excellent class of low- to medium-temperature thermoelectrics when m = 10-20. We show that the as-cast polycrystalline ingots exhibit degenerate p-type conduction and high maximum ZTs of 1.2-1.4 at 650 K when m = 6-20. Because the ingots are found to be extremely brittle, we utilize spark plasma sintering (SPS) to prepare more mechanically robust samples, and surprisingly, find that SPS results in an undesired change in charge transport toward n-type behavior. We show this unanticipated transition from p-type behavior as ingots to n-type after SPS is due to dissolution of secondary phases that are present in the ingots into the primary matrix during the SPS process, resulting in a transformation from an inhomogeneous state to a solid solution without any observable evidence of nanoscale precipitation. This is in sharp contrast to the seemingly similar AgPbmSbTem+2 (LAST) system, which is heavily nanostructured. The SPSed NaPbmSbTem+2 is doped p-type by tuning the cation stoichiometry, i.e., Na1+xPbm-xSb1-yTem+2. The optimized compounds have very low lattice thermal conductivities of 1.1-0.55 W.m(-1).K-1 over 300-650 K, which enhances the low intermediate temperature performance and gives rise to maximum ZT values up to 1.6 at 673 K as well as an excellent ZT(avg) of 1.1 over 323-673 K for m = 10, 20, making Na1+xPbm-xSb1-yTem+2 among the highest performing PbTe-based thermoelectrics under 650 K.