Speaker low crossover

First of all, reducing the level of the rest of the range will not, can not, deliver MORE bass. It can change the proportion but not increase the amount of bass. If your goal is that thundering bass, reducing the total amount of sound will not achieve that.
Alternative speakers that are more effective is one choice. Simple but not cheap.
If all that you have access to is the speaker leads, what sort of system do you have? What is the reason for not having more access to the rest of the system??