the box has a LOT to do with it. in a sealed box, in a car, you'll go flat to 9Hz with cabin gain making up for natural roll-off. in a vented box of course, you'll roll off steeply below the tuned port freq, but that'll be in part to the infrasonic filter, too.
the W7 subs will go as deep as you want them to, below the range of human hearing anyway.