I just realized that my alpine Headunit is set with its own subwoofer channel, crossover point, filtre, and gain control, but I still have the button pushed in on the amp for the lowpass factory crossover. Is that overkill, or does it not really effect sound quality? if the signal is already clean and low I should disable the xs crossover??
Let the HU handle everything just to make sure your roll off is the same rate and the same alignment/crossover type (LW, BW etc). If you filter your low pass at the HU and again at the amp, technically you are creating a steeper rolloff for the low pass stop band. The stop bands above the low pass band and below the high pass band combine when playing to essentially add up to the same level as the pass bands. If one curve is steeper than the other, in theory you will end up with a hole in response in near the crossover point. That's why steeper curves are theoretically better, if you can take away an audible stop band than you can pretty precisely control which speakers play what and take out the guesswork. If you start throwing in different alignments and stuff it just gets more complicated. Best bet is to let one unit handle the dividing and to do so as close to the source in your signal chain as your setup is capable.