We investigated the scanning differential heterodyne microscope response for a plasmonic metasurface-dielectric film system. Superresolution effect for microscope phase response with 6.5 superresolution coefficient in the vicinity of π optical phase increment was observed. Use of an additional dielectric film allowed local adjustment of the optical phase increment over a wide range (0 deg to 100 deg). The ability to adjust the width of the phase response function by changing the microscope heterodyne frequency has been demonstrated experimentally.