03 frontier cc questions

i just got my new 2003 frontier crew cab. im looking to do a few things to it. first i would like to put in a sound system. what is the biggest sub(s) someone has put behind the back seat. also where did you get the box? and how did you get the door pannels (back and front) off to replace the speakers. next thing i'd like to do is put dual exaust on it, but im not to sure how it would sound. any suggestions on both ideas.