Replying to Speakers

So I got a 93 jeep grand cherokee that I put a new pioneer deck in and two infinity speakers into the front doors but im looking to put 2 more speakers into the back doors that would possibly put out more bass than the infinitys do. I dont want to put in a sub cause I need the room in the back for equipment and shit. I went down to this stereo store of some shit and the guy was just a complete jackass and didnt know what he was talking about so i was just wondering if any of you know of some good bass speakers that would go in the doors.