The crossover in a speaker is designed for a specific combination of drivers, so you can't replace the woofers without designing and building a new crossover circuit. Three-way speakers like yours are difficult to design, especially on a budget.
The best way to get more bass is to add a subwoofer. Some good, cheap ones are the Dayton Sub-120 and the HSU Stf-1. Unfortunately, most cheap full-range speakers aren't really full-range and offer little advantage over comparably priced bookshelf speakers.
