Components/volume/audio things

[Niall]

First, you need to know the rms (not peak. Will be the lower of the two readings) power of both sets of speakers. Generally what you would do is turn the gain down to minimum on both channels on your amp, turn the head unit volume to full then crank the gain up on the amp till you get distortion then knock it down a touch.
If you have a sub, as Matt said, using the low pass filter on your amp will give you the illusion of louder clearer sound on your speakers whilst the sub does what it does best and rattles the doors off!