I have heard the kappa's in a civic as well and was really surprised with the bass response. The speaker placement in our cars is not very good. A small sub will help with this, even a 8 in will help to get the sound you want with out taking up to much space.
Other ideas:
The amp: RMS Power @ 4 Ohms: 35W x 4 @ 14.4V
Your cars electrical system will probably not be running 14.4 volts all the time. It might peak to that voltage which will allow the amp to produce the numbers above.
The Speakers: Recommended RMS Power: 100W
You might get that punch your looking for if they had more power.
