it entirely depends on what model of subs you plan to use, how much power you're going to feed them, and even more importantly, the enclosure that you are going to be using. i can assure you of this: i have heard a single 8 that has beaten a lot of dual 15 setups, so not put all focus on "size vs. size". there is a lot more to it than that...

 

I had my sysytem turned all the way up today to see how loud it gets on the outside, and noticed my back glass flexes toward the bottom, and also my cab and bed. I think im gonna get 2 more of these 15"s and a lexan back window... I'm just going to make the traditional style box, nothing fancy. But it will be taking up my whole ext. cab and to where ill have to scoot both seats up. ill probably save up for ether a Memphis 1000d or a Rockford 1200 watt. Ive heard good things on the Rockford amps, but my brother is running a single 1000d on his 4 Memphis 15"s and it powers them very well.