The polarization-dependent transmission of a basic anisotropic feedback system (Fabry-Perot resonator) is mathematically modeled by means of the Jones-matrix formalism. Detailed numerical simulations of the resonance case are performed. Small phase anisotropies as well as small polarization-dependent losses of the resonator components can be extremely amplified by resonant feedback. The amplification factors depend on the magnitudes of amplitude and phase anisotropy and their mutual interactions as well as on the polarization-independent system parameters (forward transmission, system feedback). However, for higher phase anisotropies, saturation effects occur and, therefore, the anisotropy amplification factors decrease. Our experimental investigations applying anisotropic Fabry-Perot resonators in different ellipsometer systems confirm the predicted amplification of phase and loss anisotropies in resonance operation.