hmm your truck is a 4 door supercrew right? I bought an
LMI seat lift which raised the rear seat 3". Then I installed my BM12's under the seat. They have around 1.5 f^3 each.
Here's a photo of my box out of the truck before sealing:
installed:
I think if you did something like I did along with your amp rack and a pair of mags you'd really be thumping!