Think of it this way....you do the sub now.....later on you realize the other speakers are made from pre digested food.....
Just add on amps and speakers as needed with either of those.
DSP will help by getting rid of the Bose (no highs, no low, must be Bose) equalization to make the really poor drivers sound a little tolerable.
For "just a sub" add on you could tap into channels 5 (left front door) and 7 (right front door) with an amplifier that has high level inputs, set the amplifiers crossover frequency to 80Hz Low pass, add sub........but you will end up with time alignment issues, it may sound ok.
Its always better to "do it right the first time" than to have to re-do it again...........................