i have 2 12" mtx subs. some older ones, and a 500 watt alpine amp. this set up worked in my brothers s-10, and in my blazer. then i put it in my cavalier and started having problems. it worked for a bit then it would only work a for a few mins then blow fuses in the amp. there's one 30amp fuse in the amp. i found that if i unhooked my right sub then it would always work with the left sub. as soon as the other sub was hooked up then i blew a fuse. it 2 channel and i had it bridged to both subs, now its just bridged to one sub, and it starting to blow fuses again with just one sub hooked up. are my subs finally pooched? or is there something i'm missing here? i swapped rca's and tried different grounds, checked the speaker wires, and adjusted my gains and all that. i dont know whats happening.
see if turning the gain on the amplifier down stops the fuses from blowing. you're probably causing the amp to clip by overdriving the input stage, caused by the gain being turned up too far. also make sure you have the amp grounded to a good, sanded paint-free metal chassis ground, and that you have at least 8 gauge wire going directly to the battery for positive on the amp.
If none of that seems to work, check and make sure your Subwoofer wiring is configured into a 4 Ohm load and not something else like 2 Ohms or 1 Ohm.
If the configuration seems ok, try switching the current Subwoofer with the one that is currently disconnected. If the Amplifier is still working fine, then it has to be a Setting on the Amplifier or a Positive/Negative Cable issue. If the fuse in the Amplfier blows, then it might be a Subwoofer issue.
Joe you forgot to figure for class D amplifier efficiency. 500W would be about 50A. 300W about 30A peak continuous. Since a fuse blows faster, the more overcurrent passed through it, those ratings should be sufficient, even for brief spikes in current demand.