Need help with xfl 15's box

[kfx400kid]

Ok so i have tried a few diffrent boxes and yet to find one that i am happy with, i talked to bob at american bass and was told the xfl's need anything between 2.75ft3 each to 3.5ft3. My current box is 39wx20hx22d with subs up and port to the rear in the middle of the box. This is in a toyota highlander suv. The port opening is 5" by 17.75 deep. Does this sound like a good box for them? i'm trying to get around 33hz to 35hz tunning. Is it true that with a stiffer spider that the box needes to be smaller and with a softer spider they need to be bigger. If anybody know's that xfl's then they know that they are really stiff and is that true. Aalso i listen to alot of rap music and i compete once in awhile playing music not burping. I have hit a 144.0 with my last box and a vfl150.1 and would like to get alittle more. trying to get to about a 148.0 to 150.0 with just a better box...... thanks alot to anyone who reply's.......