Originally Posted by e3NiNe
when the amp goes into protection mode, it usually means one of two things ...
1. the amp is fried or overheating.
2. the speaker drawing the power is busted or too powerful.
Since you've tried a different amp, try a different sub.
Another reason could be the wiring of the sub. If you have a DVC sub, it would be wired incorrectly and somehow you have too low of an impedance, causing the amp to go right into protection mode.
What subwoofer is this? Exactly how is the subwoofer wired? How is it wired to the amp?