I have a Fi BTL 12 running with a rockford fosgate amp but I've been wondering what I'm supposed to set the frequency knob to on the amp near the gain.I think it goes between 80hz-300hz. I dunno.any info anyone?

that's a low pass filter. What you set it to will prohibit frequencies above it to play as loud. It will gradually "rolloff".

So, you will need to set it to around 80hz. Anything higher exhibits unusual sound as subs aren't really meant to play in this range while already playing the substage. Not only that, but in this range, it also starts forcing your amps to draw large amounts of current to produce those higher frequencies that are already not as audible as 30-60hz normally is and puts an unnecessary load on your electrical system.

that's a low pass filter. What you set it to will prohibit frequencies above it to play as loud. It will gradually "rolloff".

So, you will need to set it to around 80hz. Anything higher exhibits unusual sound as subs aren't really meant to play in this range while already playing the substage. Not only that, but in this range, it also starts forcing your amps to draw large amounts of current to produce those higher frequencies that are already not as audible as 30-60hz normally is and puts an unnecessary load on your electrical system.

Ok now on my headunit I have it set at 50hz and for music like slipknot or metallica I have to switch it to 80hz.

that is a bass boost. On your headunit, whatever you set the frequency to, if u "add" anythign above 0, that frequency will be centered upon so by boosting it, surrounding frequencies around it will also go up.

Think of the hyphens below as a frequency range between 20-100hz

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Now boost 50hz by +5 and it will look like this- (periods mean nothing, just look at the hyphens)

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You shouldnt need to use this because it will not allow your response to be flat and that's what you want.

By peaking your response will likely induce clipping and clipping can over time and even during very short periods(seconds) cause a speaker to fail due to overheating, over current, over excursion, etc...