Forget 600W. Your stereo will never ever produce 600W. Average efficiency of speakers is around 95dB with one watt of power. That is a lot of noise. These things are notorious for overrating and there really is not any standard for rating. High quality home stereos are rated in RMS power usually into 8 Ohm load with stated level of distortion (THD). The only way to get any idea how much your stereo consumes is to measure DC current at 12 V at average volume and at high volume. My guess is it will be less than 3A. Multiply average DC current by hours of use to get rough estimate of ampere-hours(Ah) it will take from your batteries.
