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I got a quick question for anybody who can help. I have A Sony CD Head Unit and It Has a 125 Hz LPF(Low Pass Filter) on it. Should I use it in conjuction with the 80 Hz LPF on My AMp . What would The Diffrences be?

 
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Won't make a difference...just use the LPF on the amp..

 
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If you have the LPF on the amp set lower than the LPF on the HU then the LPF on the HU won't do anything. If it was the other way around then you could adjust the LPF using the HU instead of using your amp. It's a matter of convenience.

 
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Wrong it will affect it, you will end up having a larger slope. If one os 18db per octave and the othr is 18dc per octave you will end up having an 80hz corssover poitn at 36 db per ocatve, but you will have a 18 db per ocatve gap between 80-125, I would leave both on, provided you have your mids running at 80Hz and above not 125, or esle you will be missing alot of mid bass.

 
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Abstract is correct. It will act as a cascading crossover. It won't hurt you to leave both of them on.