If you're starting from scratch and planning on putting new stuff in I would not bother with bass blockers. Only use them if aren't planning on doing anything else. A lot of the better headunits have built in crossovers this way you can stop the lower frequencies from even reaching the amplifier and wasting it's amplification. An EQ can also do this. A good set of crossovers work well too, there are passive and active types. Car audio bass blockers (even the expensive ones) are rather cheap and although dampen low frequences at a certain dB/Octave value will still distort all frequencies that pass through it. Plus any work your amplifier has done has now been wasted. Think of your audio system as a stream of water. The more things you put in it, the worse it gets. Bass blockers are really only a temp or final solution, if you're buying new stuff, plan your system out ahead of time.