Skip the Stratus III. It is a great sub for what it costs, but is a closed box design and does not go nearly as deep as the Storm III or the Stadium III (which is deeper still).
I am not sure what Karls is saying. All REL subs have High-Pass adjustments. Their high-pass side is what makes them great for audiophiles. If he is running off the amp speaker terminals, then he is running high-pass and I assume he adjusted it to blend/integrate it with his system. (Maybe not?)