New Speakers now looking for Subs

[MiLo]

So i just bought these 6x9 speakers for the rear deck and these 4x6 speakers for the front doors, but now im looking for subs to compliment them. ohh if this makes any difference i have this head unit.

i have the slightest clue on subs and amps so i'm looking for a little help.

i know the speakers i bought are not gonna push all the bass but at the same time i don't want to get a crazy set of subs that will knock my ear drums out. im not looking to be noticed while getting to work and back, i just wanna notice my music (ya feel me). i'm looking for something simple that's 1.)not gonna take up all the room in my trunk 2.)get me pulled over 3.)not gonna get me jacked cuz haters will be haters especially living here in ChIraq lmao.
so that being said, i was leaning towards maybe one 10" or two 8"s. or maybe even something with an amp already attached. idunno what they're called. a powered sub i think.
for all you Audio Techs out there, any imput would be of great help. thanks