low voltage could throw it into protect... either because of the low voltage directly or because of excessive clipping when the voltage is dropping.. something could be shorting out somewhere... I'd say to make sure the tinsel leads arn't shorting out on eachother or the basket, but if it's a DD it shouldn't have any that can short out.. if the coil is scraping inside the gap that could cause it to short..

otherwise, it sounds like the amp just dosen't like the load the sub is giving it.. check the resistance of the coils with a DMM to be sure what they are... if they are 2ohms each they should give a DCR around 1.5 ohms...

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SIC Audio