2015 GMC 3500, Bose, Adding Aftermarket Sub

Hey guys, Having a tough time with this.. I've asked the local audio shops and even had best buy take a look, no one could figure it out. The amp turns on and the sub plays, but doesnt seem to get any louder than the factory amp. Everyone that has heard it agrees, something isnt right. It sounds sloppy at best and again, not a ton of volume out of it. Adding a 12" JL sub to the truck. Hooked up new amp to the rear speaker wires to RCA, no LOC as they aren't needed for the kicker amps. (We did try LOC too, same issue) Tried front door leads also, same issue. I was thinking this is a crossover issue as it seems to not want to play low notes. THe truck has a factory Bose system which I believe "filters" the sound as to not blow the speakers or cause distortion. Another person had mentioned they had the same issue and ended up using a L2CI LOC and that solved it. Any thoughts on this? At this point it will be a $100 gamble with the L2CI as the local shop wont return it if I open it and not convinced this will take care of the issue. Any help is appreciated! Thanks!

You must catch the factory subwoofer wires. The bose amplifier is located on the rear wall of the truck on the passenger side of the vehicle. Subwoofer Speaker Wire Positive Blue/Gray Negative: Gray/Black

Also I believe that truck has a noise cancelling mic in the headliner that needs to be disconnected and it does do bass limiting on the factor amp so a simple processor like an audio control lc2i is recommendeddouble-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer

Hi guys. I'm looking to do a blow through type box for my truck, I got 2 Hifonics BRZ15D4 Brutus 15 Which require 4cubic ft each. I'm not sure how big to go with the box because confused with the port volume and woofer displacement. I want to tune to 32 hz. I don't know if you calculate the volume of the box then subtract and volume of the port or how would that work?