I would suggest sound deadoning the doors and then fitting a small 8 inch and box and amp
This would give you a lot more bass, the Alpine unit (Same as mine
)) has a dedicated sub out so you can tweak to your hearts delight and the H/U has enough power to give your fronts an ok sound
You could do this for about £150 I reckon if you stick to cheap or second hand stuff
As you are just looking for abit of bass, not a SQ install and are not too up on fitting maybe an aftermarket bass box/amp combo would be a good investment?