I am pretty serious about doing a blowthrough to my F-150 in the next few months.  I am pretty settled on 2 18"s on 4000 watts (will be more with upgrades in the future.) I was thinking of a 4th order bandpass box for them but I want to be loud on low hertz (like around 30-35).  How can I accomplish this?  Also, what would be better: 2 18" or 4 15" all on 4000 watts?  Like I said, I want to be loud but hit low.