I would take advantage of the acoustic loading attributes of being able to put a sub in the boot / hatch area of the vehicle. Underseat woofers in my opinion are a last resort when vehicle restrictions dictate. The Focal underseat active woofer may be a very good unit when comparing it to other underseat options. There may be a good reason why your mate is selling his Focal iBus, and that may be that he was underwhelmed with it's performance compared to other subwoofer combinations.

Do yourself a favour and visit some reputable car audio shops in your state.

Oh, and for a headunit I would choose an Alpine over a Pioneer.

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