“My initial thoughts are the GroundMaster, coupled with the GoundMaster City (no inclination to attempt driving an 8ft copper rod through my garden soil).”
OP, do you mean the RouteMaster coupled with the GroundMaster City? I have that combo, but not the PSM156. I believe that you can connect the PSM156 to the RouteMaster, along with your other components. From what I understand, however, there may be some benefits to running a separate GroundMaster City just for the PSM156. Best to check with your dealer, or PAL directly.