Is your box a pre built from a factory? I'm not sure I would recommend a larger amp for your subs without knowing more about your enclosure. Of course you could run a bigger amp but it may just destroy your subs. Can you do a little research or take some pics of your set up? Need to know your box specs, what size wire your running, alternator amperage and type of car.
Also it sounds like your running your door speakers off of your head unit. You can defiantly add an amp to these but I recommend running new speaker wire to each one. But first lets see if we can get some pic of your set up, then we can get you set up correctly.