I decided to go with Kicker Comp C-10's because they were really cheap for the quality. I wanted to put in some JL w7 10's, Kicker solo-barric L7 10's, or Rockfos Hx2 10's- but mostly, im in highschool and I cant afford it. The C10's are nice subs, and they pound really hard, but I wouldnt recommend my set up if you had anything other than a singlecab truck. Kicker's are really space hungry, so it would have been a lot easier to go with rockfos's or JL's, but again, its all about cash. Facin em down, or "Down-firing", allows better bass displacement *if* you sound deaden correctly. exteremly low tones travel unlike higher frequency notes, they disperse much better, and much more evenenly. If your sub is "center-fire" or facing up, your seat coushin is absorbing probably 30-40% of your output, not to mention, the bass doesnt have a unform stage to disperse on, which means at the same volume, a sub that is downfired is going to be way more efficent than one that is centerfired. Im by no means an audio expert- I only know what I've read. But if you ever have your seat out of your truck, block it up off of your carpet about half an inch, put the seat back over it, and fire it up. Even with out sound-deadening, chances are its going to hit harder than it did facing up. Not to mention, it wont vibrate your ball's as much
Go to Sounddomain.com and check out the forums, theres a ton of stuff to read, and a lot of helpful people that know WAYY more about it than I do.
Also, what guage wire, what kind of alt. are you using, and what sub/amp? Ohmage?
-Tanner