For bass you need surface area. You're likely not going to be able to coax much more of it out of your existing speakers. If your speakers are high quality, it's possible your head unit doesn't provide enough power for them to boom. But in most cases you'll want to consider a sub& to get the bass you're looking for.
It's also possible that you've blown your speakers and there isn't much left of the bass cone. If they're stock or just old, they dry out and crack over time.