If upon investigation it shall be found that any regulation, measurement, practice, act or service of any public utility complained of is unjust, unreasonable, insufficient, preferential, unjustly discriminatory or otherwise in violation of law or if it be found that any service is inadequate or that any reasonable service cannot be obtained, the Commission may substitute therefor such other regulations, measurements, practices, service or acts and make such order respecting, and such changes in, such regulations, measurements, practices, service or acts as shall be just and reasonable. The Commission shall investigate the promotional allowances and practices of public utilities and shall take such action as such investigation may indicate to be in the public interest.