The DSP is a tool used to obtain the flattest response (and go low with the same +/- 1.5db) at a lower price (after a 5 minutes search, I did not manage to find back the post from Ian talking specifically about that).
The flattest the sub, the easiest it becomes to place in a room (unless you just happen to have a room that compensates exactly for a non-flat sub; it can happen!), and even more so when you want 2 or more subs as they will blend more easily. If there is problem in the room, then you can use an equalizer to further compensate.
