@ ailiahaider
ok here is my advice. do get a capacitor as it would add quality to your sound. and the battery too as it will resolve your lights problem and more current would flow wich would in turn give good quality to the sound.
asuming that money is no problem for u, then i think u should take the woofer off this amp and get a singel channel or mono amp, as u put it, for your woofer. and i think 800watt would be good, the bass would be good but not too much. and for this 800watt amp i would sugest "Solidaudio", cous i have a great experince with. and another thing some one told u to change your woofer to sony explod, man dont even think about that. sony explod is a shit woofer. it does produce a really loud boom but has no qualit and i know u want quality.
and as for the existing 4channal 1200watt amp u have, buy anther pair of kendoow speakers for the back. like this u would have 4 speaker at the back. and then connectall the speakers to your 4 channal amp.
and if u r doing so much then also change the shity pree u got in your liana. coues pree is also a major factor for quality sound. and u should go for the top of the line pioneer pree as those thing rock. i guess it would be round 10-12k but is worth ewery peny. and plus u can switch the woofer on and off from from a pioneer pree.
and if money is a problem, then just switch the bass boost switch off and chill, mashalla your existing system is also great, except your pree.