This article studies the exploitation of the nonlinearity of terahertz photomultiplier tubes (THz-PMT) in conjunction with an independent injection-seeded terahertz-wave parametric generator (Is-TPG) to achieve high differential sensitivity. Si-wafer resistivity measurement and water vapor spectroscopy applications with an independent Is-TPG source and THz-PMT detector are showcased. THz-PMT spectroscopy measurements are compared to pyro-electric detector-based measurements to highlight the tube's high differential sensitivity performance in THz spectroscopy. THz-PMT spectroscopy with a large contrast ratio more than two orders of magnitude larger than that of conventional detectors make this THz technology very attractive for a wide variety of spectroscopy and measurement applications.