Settings: Headunit vs Amp

Hey there
I have a 12" sony sub and amp specs as follows

AMP
Max Power (Watts): 1200
Ad Hoc Power into 4 ohms, THD%: 202
Ad Hoc Power into 2 ohms, THD%: 250
Bridged into 4 ohms, THD%: 500
Signal to noise ratio (dB): 93dBA
Frequency Response (Hz): 5-50kHz
Input Level Range (V) Line level input: N/A
Input Level Range (V) Speaker level input: N/A
Operating Voltage (V): N/A
Current Drain at rated output (A): 48A

SUB
Speaker Size (cm): 30
Speaker Size (inch): 12
Peak Power (W): 1300
Rated Power (RMS, W): 380
Impedance (ohms): 4
Sensitivity (dB/W/m): 89
Frequency Response (Hz): 18 - 2,000
Mass (kg): 4.5
Mass (lb/oz): 11
d (mm): 250
d (inch): 9.8
Rdc (ohms): 3.3
Fs (Hz): 24.8
Qts: 0.37
Qes: 0.39
Qms : 7.02
Vas (Lit.): 86.7
Vas (Cu. ft.): 3.06
Mms (g): 162.1
Lvc (mH): 1.1
Xmax (mm): 5.7
Xmax (inch) : 0.2
BL (Tm): 14.1


I was just wondering what the best set up is for these components, I think i am running the amp "Bridged into 4 ohms, THD%: 500" connected directly to the +/- on the sub. The amp has three settings LPF, Gain, and Low boost
If i put the lpf at 80hz, turn the low boost all the way down what should my gain be at, i usually have it at 1/2 way to full is that ok?

In my headunit I also have a LPF and HPF if i change these filters is it gonna conflict with the LPF of the amp? or should i just turn it off?
And one more question is what is the difference between these two settings in my head unit Base -12db to +12db (can set every 2db between them) and Sub.W -Infinity to +6db (again every 2db -Inf, -6, -4, -2, 0, etc)
Thanks in advance(halfcat)

most people use either one crossover or the other but not both. The gain control is to match the output of the CD player with input of the amp.

The bass knob is like an EQ, probably set to around 50 Hz. So if you set it to +12 dB then the freq's around it will also be effected. so 40 & 60 Hz might be at +9 dB and 30 & 70 Hz might be around 6 dB or something like this. On the other hand, the Sub W. control effects ALL the freq's being sent out of the Sub W RCA jacks equally. This will require you to use the crossover (LPF) on the CD player.

Wow, understand it a lot more, system sounds great ,
thanks man!