Is this for an E46? I don't think that an eq is going to help much more than the stock's head unit bass and treble controls. They HK system has already been "tuned" to sound best in the car. There is not a lot you can get out of a relatively small amplifier, crappy speakers and two 6x9" "subwoofers". I would highly recommend replacing the whole system. If you do want to keep the stock head unit for cosmetic reasons, you could add aftermarket amps and speakers for improved sound.
If you do want to still pursue adding the eq, I believe the HK system uses differential signals (6 volts?), so you need an eq that can handle differential inputs and outputs (I believe some Audiocontrol equipment is capable of working with diff. signals, but don't know which models). You can tap the signals at the head unit or in the trunk (amplifier is located in left side of trunk).
If you do want to still pursue adding the eq, I believe the HK system uses differential signals (6 volts?), so you need an eq that can handle differential inputs and outputs (I believe some Audiocontrol equipment is capable of working with diff. signals, but don't know which models). You can tap the signals at the head unit or in the trunk (amplifier is located in left side of trunk).
