qguy's answer is the electrical means of acheiving what you want.
If you want a mechanical solution, either:
1. modify the enclosure (i.e., convert the sealed enclosure to ported in order to increace efficiency at the tuning frequency), or
2. change the driver.
for either option, you would have to know the T/S parameters of the driver in use, and then use a speaker modeling program to simulate the frequency response when used in your enclosure.
also, changing the surrounds to foam guarantees a change in the driver's parameters, but does not neccessarily guaranty an increase in the bass region. Again, if you insist on increasing bass by mechanical means, it all boils down on how the driver's parameters match the enclosure design.
My suggestion is that instead of trying to increase the bass, just lower your mid/treble level by modifying your speaker's crossover. adding a notch filter or a simple resistor network to tame the mids and highs will make your bass relatively stronger... of course, at the expense of overall speaker efficiency.