s10airide

Im running a 15" RE XXX off a soundstream PCA-2000D. I like the low tones from the 15, but as you know 15's dont have the responsivness to handle higher frequencies. Im thinking of adding either 2 eights, 1 10 or 2 10's to pick up the higher frequencies and running them off a separate amp and HPF. Any suggestions on which setup would be best? Any ideas on some good, cheaper subs that can handle this duty?. Also, I just bought 100^2 feet of 80 mil sound deadener (rattle trap). I was wondering if I should just cover my floor, side panels, doors, and firewall? or should I do the roof as well? I drive a 2001 2dr blazer so Im not sure if I will have enough for the roof and if the roof would be necessary. Any input would be greatly appreciated.