A few things can possibly help your situation
first things first, I wouldn't turn up the gain on the amp, as that is what distorts subs and can blow them.
You need to find a good point that makes them sound nice without distortion.
These are the steps you need to take to tune your sub and set the gain
Once this is done, if you still want more bass, you may need a better box that is tuned to the subwoofer. Box is everything and is key to a good bass sound. you can also get a better subwoofer that can handle more power, but you will need to get a better amplifier that matches the subs specifications (match RMS power from the sub and amplifier)
Make sure your cables are in good condition and installed correctly. Typically you want 4 gauge for anything over 350-400 watts RMS. It would also be wise to do "the Big Three" (rewiring for bigger 4 gauge battery cables from the alternator and grounding strap)
Again just make sure that you tune it correctly on both the amp, and the box, make sure your cables are good, and if you still want more, buy new gear
dont' worry about a capacitor. they don't do much good if anything at all
good luck ;)