Help with audio system

Hiya guys...
My system at the moment is an Alpine CDA 9856 and two 12 inch hertz energy line subwoofers and a hertz EP4X (see picture)
the only problem is that the alpine does not have a high pass filter which means i am getting bass through my front (standard) components ( i.e the mid cone)
obviously this is uneccesary as i have plenty of bass from the subs! :) but what it is doing is distorting the front speakers. i went down to the local car audio shop and they have sorted me out with some "bass blockers" that are to be wired in to the wires between the crossovers and headunit.
1. firstly do any of you guys know much about bass blockers? are they successful?
2. secondly, does anyone know the whereabouts of the nissan standard crossovers??
thanks alot for your time and effort guys
wilzy

also...whats the best way to compress my picture??
cheers