if you want a true system even a small ne just add even a small 10" sub in a small box and amp. door speakers arnt designed to have the low freq go through them because it takes a lot of power to make low feq audiable. if you want a little bit of mid bass get a good set of 6X9's for the rear speakers.(you can even put in a small amp to run them and then you would get a little bass)
I run 4way 6x9's in the rear(250w),3way 6.5 in the front(200w), 200w tweeters on the dash, and 2 16,000w 18' spl comp subs in the rear.
1 6channel zapco to run the mids and a high current modified earthquake D2 comp amp for the 18s