The "old" design is like having the sub-sonic filter off, so turn it off on the "new" design and you'll end up with the old. It is an additional cut off for LP rumbling (that you cannot really hear as it is below the physical capability of human ears except for newborn babies and even then) so you don't HAVE to have it on.
The fact that you lack some range mean you do not have the correct placement for your sub. Invite some friends and have them do the sub crawl (look into the video section on the "advice" secion of this site for more information on that).
Don't worry, it will get there in the end! It is not always easy to find proper placement for a sub.
