Jplaya135 :
An L7 12 would be just what you are looking for. PORTED. Even if you kept the same amp,(hifonics 1500), the 12 takes 1,500 PEAK. Buy a dual 2 ohm sub, run 1 ohm, and turn gain down. A 15" would require 1.5 cu ' sealed at MINIMAL. In my personal experience, if you're going to run L7, go ported. I had 2 L7 15"s in a 2 cu ' sealed(each). I thought it SLAMMED... Until a friend of mine built a 6.89 cu ' box ported in a s-10 (extended cab) w/ a kx1200x1 @ 1 ohm. Hit a 153. Not too shabby... It's all up to you and how you want it to sound, and at the same time have it within your budget. L7 12" here goes for $150' ish. Whichever you go with, go ported.
-Jordan