totally depends on the system. component separates require crossovers that come with the speakers. subs require a crossover, but most sub amps today have a LPF built in already. I usually suggest using a HPF for components just to eliminate low frequencies that cause the speakers to bottom out. it's totally up to you though.
You guys seem to prefer exterior crossovers over those built into amps. If its a good amp, such as a JL, how much better will it be to use an external amp instead?
Active Crossover: 12dB/Octave, variable 50Hz-200Hz, selectable high pass, low pass, or all pass;KX100.2 hig or low fixed @ 80Hz would that work for a low pass cross over?