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Higher-order-mode dampers were recently installed in the storage ring rf cavities at the Advanced Photon Source to reduce longitudinal coupled-bunch instability. It was discovered that the 535-MHz cavity monopole mode created beam instability at beam currents in excess of 85 mA causing effective horizontal emittance blowup (due to dispersion). Methods of compensating for the instability by detuning the cavities and adjusting the cavity water temperature were becoming more difficult at higher beam currents as tests were performed to prepare for eventual 300-mA beam current operation. Electric field passive dampers located on the median plane of each cavity were determined to be the most promising solution. Simulation models were created and verified with low-power testing of the dampers. High-power testing of the dampers as well as conditioning of the damper ceramic load were also performed at the APS 352-MHz rf test stand and compared with simulation results. Preliminary test results will be discussed. *