bass question

I've got 4 speakers and a head unit. Two 5" alpines in the rear and two 5" kenwoods in the front with a pioneer headunit (45x4). I know this is hardly an imprressive system, but there is almost NO BASS at all. I feel like Im not getting my speakers worth.

I want to spend around $100-$150 to get my system sounding alright, but I dont want any subs taking up trunkspace. What should I do?

replce your rears with a set of 6x9s

6X9's won't give that much more bass. do you have a trunk or a hatchback ?