WPLM-99.1's signal is strong in Boston itself and points south, but is weaker north of the city.
I am somewhat surprised that WPLM's owners haven't sold the station.
Were WPLM put up for sale, two possible buyers might be WXRV-92.5 (to extend their rock format to the suburbs south of Boston, Southeast Massachusetts, Cape Cod, and the islands) or WGBH (to likewise extend the classical music format of WCRB-99.5 to those areas).
I don't think FCC regulations would permit WPLM to move their transmitter any closer to Boston. If 'PLM were able to move their transmitter closer to Boston, I think they woukd have done so long ago.