My simple advice is to add a powered sub (you can build one). The separate amp and controls allow for so much more flexibility. You can dial it in to accomodate room acoustics. The trick will be getting the correct amount of bass and mid bass out of the bookshelfs to blend properly with the sub. You could try modding the crossover to do that. That project would be a lot easier than engineering 3-ways out of 2-ways.