you need to tell us more about your equipment or we can guess forever. protection can be caused by one of two things, either thermal or short circuit. thermal protection is usually due to the amp running too hard/hot because the amp is not getting the proper impedance. since you said it's not bridged, i assume you have a 2 channel amp so each channel should be wired to a sub running at 2 or 4 ohms. what is the resistance value on the subs? how many voicecoils? short circuit is when the ends of the sub cables touch one another, or if the positive end touches ground like the chassis. how is the installation...nice and neat with no frayed wire ends?