what could be causing the speaker to keep blowing?

I doubt that you're playing them too loud. If you were cranking enough volume through them to damage them, your ears would probably fail before the speakers did. Those Diamond Audio speakers are pretty robust.

If I recall correctly, the 400 series is a composite set -- a 4" speaker and a 1" tweeter, coming off a common crossover.

I would check and double check the crossover settings. About the only way you could wreck that speaker (I'm assuming it's the 4", not the tweet that is failing?) is by running too much low frequency into it. Those Diamond Audio crossovers are fairly complex and tricky to configure. If somehow you were set to low-pass at the crossover point instead of high-pass, bad things would happen. That's an unlikely scenario, though -- those door speakers would sound pretty awful if that were the case.

tanstaafl.