Hows it "contradicting"?

I'm asking if you have the available space for an IB subwoofer and if you intend on letting the subwoofer have that space all of the time.

Otherwise, get a slim 12" that fits in a small box and attach the small box to the bottom of the speaker in the stock location.

Otherwise your sound will change based on what you have in the trunk that day.

Up to you what you want to accomplish.