Great questions! I just got done helping a customer with the same vehicle, but different model (extended cab). Both of these trucks have a variety of custom sub enclosures, but sadly, not through Best Buy. I did have the chance to ask Agent Google what options exist for your vehicle (2008 Chevy Silverado Crew Cab Sub Box) and I'm showing that you have many options from single subwoofers to dual subwoofers. All of the options will be for under the back seat of the vehicle, and likely going to be 10" subwoofers (shallow mounts).
Depending on the subwoofers would depend on the amplifier and amp kit that would be needed. When you order the subwoofer enclosure that you would like, you can really get into the nitty gritty and I'd be able to assist you further with finding the best subwoofers and amplifier that would sound amazing in your vehicle. Another fantastic option would be to schedule a car consultation to speak with your local Geek Squad Autotech. They'd be able to set aside 30 minutes of time to work with you to build the perfect system!
I hope this helps get the ball rolling in the right direction. Get that sub box ordered and stop back so that I could lend a helping hand, or click the link above to speak with one of us in person in your local area!
Best of luck!
