recommend a subwoofer for my entry level setup

[extrarice311]

my current setup : amp JBL gt5 a604, seps JBL GT6c, coaxial JBL GT6 6. please recommend the optimal size of subwoofer for my setup. im not planning on getting an extra amp for the sub and would just use the amp mentioned above and have the stock hu ( stock 2013 honda city) power the coaxials. please be specific with your recomendations guys. please recommend the type of installation aswell ( under seat, trunk...) and lastly what type of enclosure. i am not looking for a boom boom setup. just want that extra oomph to complete the sound experience while driving. budget will be 8 to 10k ( includes speakers and installation cost)

thanks guys!

[Leo C.]

I would recommend not to use the amp but instead install two underseat 10" subs. Reason? I want to preserve the truck space...