What causes the doors to vibrate?

[exsterminator]

I listen mostly to house/techno music which is very bass heavy. I have an 09 OBXT with the HK system and it is crap. I know that replacing the HU is the first step when redoing the sound system but it is not straightforward to do how I would want it done, so my questions from you knowledgeable lot are these:

 

1) Is it the crappy speakers that cause the doors to vibrate when music is playing? I am only on volume setting 10 and it is heavily distorted and rattles non-stop. If so, could I get better quality speakers only to remedy this? Would Dynamat help?

 

2) Should I do front and back doors?

 

3) Can I simply replace the sub in the trunk for a better quality one and achieve any noticeable results?

 

4) At what point in the process would my HK amp under the passenger seat need replacing? Basically, if I put in new front and rear speakers and a new sub, would the stock HU and amp be sufficient?

 

Recommendations for possible solutions would be great. Would like to keep budget around $300.

 

I am not looking for a top-quality sound system but something that largely eliminates the door noise. My wife's 04 BMW X5 is so much better it is a joke and it is 100% stock.