When I start to get in the upper limits of volume, I am getting a noise from subs. For the past month I just figured the sub was maxing out. So yesterday I decided to take my downfire box an lift it up firing toward the front of the cab and listen to the sub. Well the subs sound great, in fact, they have ton's more output before bottoming out. The noise that I have been hearing is coming from an air leak between a spacer ring and box. Going to have to pull sub out and recork ring from the inside. It's a pretty impressive leak.

 

Either get some adhesive/sealing caulk and run a bead around the spacer ring to secure it, screw the spacer ring back down. then get some or get some of the foam strip that comes in rolls and run that around the space between the ring and then reinstall the sub.
No more leaky leaky.

Under seats id probably upfire if possible, youd generally want 3-4+ in of room to downfire without a lot of loss of output. I just did 2 idqv3 12's in the back of mine downfire.

 

If you do use caulk to seal it be careful which kind you use.
I've heard that you need to remove your subs completely if you use silicon based caulk because it can damage them before it's completely dry. Just a quick warning.

 

Yep, I heard that the silicon will admit fumes that will soften the speakers glue while drying.

 

Dont use it to seal the sub, use it to seal the spacer.
I normally use the flex stuff that gets rubbery.

Use the foam around the sub!