Sunny,
The absolute cheapest way to beef up the volume and quality is to add a self-powered bass amp to your rig.
Here's how you cut he corners of buying a dedicated crossover:

1) Use your main mixer/amp to power your speakers, as usual...only use the EQ to roll off the low end from about 250Hz and lower.

2) Use the monitor send to feed a signal into the bass amp where the HIGHs are rolled off above 250Hz.
This will essentially give you separate control of what goes to the tops and what goes to the bottom. It works like a charm too.

3) adjust each channel so that you get the amount of bass you desire with the monitor send on the individual channel. Voila !
Psuedo-bi-ampd!

It's so close to perfect, I'm surprised that not more people do this.