To the op.
I Also Have A charger rt. I have my box tuned to 27. With 2 xfl 12's
2K watts a piece. With a ab 150.1 box is ported toon front face. With subs facing towards trunk lid. Shit slamms. But porting I not the actual cabin would be ideal which is what I'm getting ready to do. Will be making a new box soon where the port goes through the back deck into car. Thinking about a bandpass as well. But not sure if my subs would work well I n a bandpass. Never seen anyone with xfls using a bandpass tho.