The vibration transmissibility of passive vibration isolation systems is designed to be low in the expected frequency range of the disturbances but is significantly higher at lower frequencies. The goal of semi-active damping systems is to reduce the isolation system transmissibility in the low-frequency range, without sacrificing performance at high frequencies, by controlling the stiffness and damping properties of the isolation system via feedback measurements. In this study, experimental implementation of a semi-active damping system in a very small scale lightly damped structure combines electrorheological (ER) fluids with a nonlinear analog feedback circuit. Detailed modeling of the experiments enables the estimation of immeasurable internal forces in the ER damping wall, which, in turn, provides insight into desirable stiffness and damping characteristics of semi-active damping systems. The guidance provided by these experiments is applied to the analysis and design of semi-active visco-elastic vibration isolation systems. The benefits of the controllable damping and controllable stiffness effects are compared in the frequency domain through transfer function estimates computed from simulated response to wide-band disturbances. Frequency response analysis of these numerical models for different levels of isolation level stiffness, device stiffness, maximum device damping, and minimum device damping are presented to provide guidelines for the design of low-transmissibility semi-active damping systems. The coupling of control experiments with numerical modeling illustrates that controllable damping offers more effective vibration isolation of base-excited structural systems and minimum device damping is the most dominant factor that controls the acceleration transmissibility.