new sub suggestions?

[slow240sx]

I currently got a dc level 5 18" in bandpass box on 2 crescendo bc2000 amps. I had this setup in a ported wall in my 240sx hatchback and it sounded pretty good and did just above a 150db. Now, i put it in my suburban and it just dont seem to be cutting it. I have a friend looking for an 18" sub so i might be able to sell it. I dont plan on getting more amps in near future so i would be limited to 4000 watts. I have 15 cu ft that im willing to give up. Im thinking i need more cone area cause its a lot bigger vehicle. Should i do 2 descent 15's that work with 2000 each or maybe 4 15's that work with 1000 each? Maybe even 2 18"'s that will run on 2000 each? Open to all ideas at this point. Money is an issue but if i come up with a setup i like i will come up with the $$$. I thought of getting another level 5 18" but i dont want to have to buy more amps. I feel that with 4000 watts i should be getting louder than it is.