Don't know how the amp(s) are set up in that speaker system. The amp is probably an IC-based style (not discrete components) inside the reciever unit. And don't count on a real 1000w total - it's probably rated "peak" which is at least twice what it should be rated for (rms).
The speakers are matched to the amp used. You can use other speakers instead, but have to maintain the correct impedance. Most home speakers are around 8ohms. Simply adding another set of 8 ohm speakers in parallel will reduce the impedace to 4 ohms. This should create twice the power, but the amp will probably either cut out or thermal. If you want louder volume from your main speakers (not the sub), you'd have to get more/different equipment.
If you just want to add more bass, you can probably add a powered subwoofer. You can get a pre-built one, but if you already have the kicker sub (and a box to put it in), you may be able to use an add-on "
plate amplifier" to power it. Just make sure you get one that has the power you want at the subwoofer impedance your kicker has (probably 4 or 2 ohm).