4Ch Amp For Sierra

My friend of my has a 2003 sierra.  he recently put two ported 8in l7s under his rear seats.  He likes to bump hard and when he turns up the volume the lows and Highs on his componet speakers sound distored, and he has boston acoustik componets.  He has all the componets powered from th e HU.   I told him to invest on a 4ch amp and a crossover for his componets.  Since i really haven't work with seperate crossovers i want to know what is a good crossover for the job or if the ones on certian 4ch amps are good enough.

Now i know he needs at least a 12db/octave crossover right.