7)SOUND SYSTEM: Car has OEM sound system with JVC CD-player having bluetooth and aux connectivity + 4 speakers
FOUND THIS STUFF ON YOU TUBE...
1)SOUND DIGITAL SD2000.1(smd)
2)Subwoofer Pioneer 12 1400w/400 Rms
NOTE:I HAVE A KENWOOD SUBWOOFER 1000 W BUT DONT HAVE AN AMP SO I NEED A 4 CHANNEL AMP .ALSO EXPERTS PLZ SUGGEST OTHER STUFF REQUIRED TO ACHIEVE SOUND QUALITY WITH AMPLE AMOUNT OF BASS.
What is the budget you have for audio?
The upgrade you mentioned, did you figure this stuff out from a Youtube video 
SD2000.1 is a mono amp pushing out almost 2K Rms when powered properly, The 400 Rms pioneer sub (308, 309) cant take that much power alone. You need to change your head unit first. A good set of comps and amp for them, decent mono matching the rms of sub.
If you state your budget then it will be best to advice