The fuse on an amp only blows if the current across the speaker wires goes to high. Typicly this is caused by one of there problems. In the order of most common:
1. speaker wires are shorting out outside or inside of the box, possibly at the terminal cup.
2. The sub is toolow of a resistance for the amp to handle orthe sub is blown and is basicly shorted out across its own voice coil.
3. The amp is shorted out internaly.
The fuse on an amp has NOTHING to do with the power or ground wires. Only the speaker wires.