thanks for the input.

the reason of preventing bass coming out from speakers is that the front speakers make the front door vibrate and producing very annoying sound, and rear speakers bass is making rear glass "dancing", typical Proton quality 

if really the connection between amp and hu is wrong, means i have to take out everything and reconnect? 

omg......

to be honest, i myself also dont know what kind of bass i am looking for...... 

i listen to a lot of trance and new age music, how to describe the bass i really got no idea, sorry.

what i looking is just "filter" out the boom boom sound from my door speakers and rear speakers, i tried to set FLAT on the HU EQ, its goes flat, but then the sub also going flat as well 

as for sound proof, i personally dont think its worth it to install on a 8 years old car? 
and furthermore my setup is just very basic and cheap, not high end stuff or what 

anyway, thanks to all Sifu once again for the great input, really appreciate all the comments 

wow! i will definitely jot down this on a paper, very very detailed explanation   

will try to fiddle with all those settings that u mentioned this weekend when i am completely free 

...
UPDATE: 22 / 11 / 2013

ok, so now the problem consider solved, but still, due to my own curiosity, i try to digest what did u all audio sifu out there told me last time, especially forumer 6UE5T, and i also have been google-ing for quite some time regarding the settings of HPF, LPF, Gain, Bass Boost etc etc. maybe due to all these, i still have some doubts in my mind now.

Here is the amp setting which i NEVER touch / modify / change since the day of installation:
[attachmentid=3735570]

GAIN:
as far as i know, gain cannot be too low or too high? too low means not enough power for the amp? and too high means can over-powering the sub and damage the sub? for now i leave it at the center mark, dare not touch. 

Gain is actually setting to match the volume of your HU. Usually you'd need an installer with voltage measurement to do gain matching between your HU & power amp. However the easy way to DIY even though not accurate is to first turn it to minimum gain. Then turn your HU volume to about 75%, then while paying music start turning the gain on the amp slowly until you can get the loudest sound BUT still clean/without noticeable distortions, then stop it there. Do this while all bass/treble/EQ/loudness settings are flat.

BASS BOOST:
from my reading through google, majority of audiophiles and audio sifu never recommend to use bass boost, many will say better to leave it zero because bass boost will "pull" lots of energy from the amp, dangerous and will cause overheating, and also make the music sounds artificial. and yet some will argue that bass boost is fine to use as long dont use it too high level. so again, i leave this at center. 

For me, it's all up to your ears, set it to the level that you like the most,  coz in the end it's you who listen to it, not other people. But since this is at the amp whereby you cannot adjust it easily (unless the amp has a remote bass boost controller), then better not set it at max, usually medium. This way you can till have room to to adjust using your HU bass controller.

LPF:
ok, this is probably the most confusing for me. the LPF marking on the amp only stated 50Hz and 250Hz, so i assume that setting the knob to exactly center means 150Hz? 
if it really means 150Hz, how about the LPF on HU? i set it to 80Hz, output level +3 and slope level -12. so which is the real LPF? 150Hz on amp or 80Hz on HU?  or i will end up getting something in between 150 and 80?
and is my output level and slope level set correctly? i dont quite understand what is slope level even after fiddle with it.  

If your HU LPF is at 80hz and your amp is at 150hz, then the one that will take effect is the one from your HU as it's lower frequency. For me usually I'd set the LPF on the amp to completely off meaning no LPF if the HU already has LPF, so that I can completely control the LPF just from the HU which is easier to adjust. If the amp cannot completely deactivate the LPF, then I'd turn off the LPF on the HU, so the one that will be active is on the amp and I'll adjust the amp LPF until I got the bass I like. The aim is to try make just one of the setting to be the LPF control. If both cannot be turn off completely, then Id make the amp LPF to the highest frequency (in your case at 250hz), so that the HU has more margin to adjust LPF below 250hz. In your amp above, I'd set it at full meaning all LPF & HPF are off. As for the slope and level, again it's up to your ears to determine until you get the bass you like. I usually would set the slope as steep as possible so the cut off is cleaner.

HPF:
since this amp is for sub, and the Mode already set to LPF, so the HPF don't need to bother right? its alright to just leave it like that?

Yes, if you only connect the sub to the amp and no satellite speakers then the HPF on the amp doesn't matter. You can turn it off by selecting FULL.

my current HU settings:
HPF set to 125Hz, slope level -12. i came to know it is better if LPF and HPF value are able to be as close as possible, but due to the super noob quality of stock Gen2 speakers, i have no choice but to set 125Hz, or else the front doors will start to have weird noise. so now assume that my LPF is 80, and HPF is 125, so anything between 80 to 125 is not fully covered? this condition is consider OK?
Bass Boost = 0 (because unnecessary?)
Loudness = Max (because Off, Low and Mid sounded weird)
EQ = Vocal (still prefer Vocal more than the rest)

Yes, in your settings above, it means between 80-125hz is not fully covered. It doesn't mean those frequencies are completely gone as they can still come thru but at a much more lower/muted level. Ideally it should not be like that as you want the full frequency spectrum to be able to go thru equally. I'd guess that the bass guitar and drum toms sound will be more dry/not enough humming. One way to alleviate this a bit is by making the LPF & HPF slopes less steep, hence they would overlap more. Again this is up to your ears, which one will sound the best for your with the equipment you have now. Same with bass boost, loudness, and EQ, which ever sound the best for your ears. I personally often would also turn off loudness and rely fully on the EQ.

i know i am being very fussy here, but if there is still any sifu or audio pro willing to answer my miserable questions, please feel free to do that. million thanks in advance! 

thanks once again, for the very kind help of yours     

now finally no more misery for me 

i made screenshots of all useful tips here, and then print out hardcopy, keep in my car