Ok with my Pioneer Premier DEH-P470MP HU I have HPF at 50Hz, 80Hz, and 125Hz. I do not have an external amplifier on the speakers so what should I set the HPF to, to maximize the output. Also, what should I set the LPF on my amplifier to for my subwoofers? It seems even at 125Hz I can still hear a little bass coming out of the speakers. I have had some people reccomend using the 125Hz on the HU and an 80Hz on the amp but aren't I going to be missing frequencies in there? Thanks for the help!
Start with 125hz for the speakers, 80hz for the subwoofer(s). Remember that a crossover doesn't instantly eliminate all frequencies below the filter, it will gradually roll off at a rate (6db/oct, 12db/oct, and so forth). If you have two speakers playing the same frequency, you're adding surface area, thus volume, so depending on the crossover type, you would have a boost at the crossover frequency due to both drivers playing it (in your case, the subwoofer and the components). So, you set a gap between them so that the rolloffs of the drivers will match and the end result will be a flat response.