A system (10) and method (50) of equalizing an audio signal from an existing radio or head unit (30) using an interface unit (10) between the radio or head unit and a replacement load (34) can include the steps analyzing (64) an existing equalization to provide a measure of an existing frequency response, generating (66) a substantially inverted frequency response to the existing frequency response, and applying (74) the inverted frequency response to the replacement load to provide a desired response. Such as system can include an interface unit (12) having an automatic input level control unit (14), an analog to digital converter or digitizer (16), a DSP (18) providing an output to a DAC (20), a level booster unit (22) and a processor such as a microcontroller unit (24).