I lowpass my JL 12W3V2 at 100 Hz and highpass my Boston Acoustics 6.5 Proseries at 100 Hz. Every setup is a little different, but for me the sub bass is perfect at these settings. I also have my bass boost setting (which provides more peak 40Hz levels) set at 12db.
I am using the crossovers built into my amp (Kicker KX600.4). Most aftermarket amps have the high and lowpass crossovers that are adjustable. I also have crossovers for my BA components but now we are not discussing the subwoofer anymore. The lowest I would set a sub is lowpass at 80 hz. If you buy a good sub, why waste the bass frequencies that it can produce? Most subs frequency range is between 20 Hz and 200Hz. Don't waste a good portion of its range so that you only use it from 50Hz to 20 hz. Thats what the bass boost setting is for.............to give you harder bass punch at 40Hz. Most peeps will low pass between 75 hz and 125 Hz. Its up to you.
Look at the frequency range spec for your sub.......and use what is good for your total system. I have some good midbass drivers with my Boston Acoustics components, so I know that they can produce 100 hz with no distortion at very loud volumes. If you do not have high end speakers for your midbass, then let the sub handle some of that maybe even up to 125 or 130hz. The lower in frequency that your midbass drivers go the more likely they will distort at higher volumes. See how your system sounds together and adjust the subs lowpass crossover where the system sounds the best at high volumes.
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