Hello I need some help car audio gurus! I'd like to add rear speakers to my sound setup. This is my current system:

Kenwood KDC-2020 Head Unit Maximum power 50Wx4
Targa X series front speakers w/ tweeters
JL Audio 12" W3 Car Subwoofer

What should I add? Some suggested to get a 4-channel amp to connect both the front and rear speakers. But I don't know which type of rear speakers to get.
Some also suggested to get a 6x9 coaxial oval rear speakers.
Do I need an extra amp for my rear speakers or can I just get rear speakers and connect it directly to my head unit? I have a budget of 6-8K so which ever setup will make my audio system sound and feel great then I wil go for it. I'm only a newbie and I'm really not that famiiar with sound setups. Pinasetup ko lang sa friend ko tong curent systme ko ngayon. Gusto ko lang mag-upgrade ng konti. Please help. Need your advice masters! I used to be SQ but now I fell like SPL... I'd like to increase the loudness/volume to the next level but not sacrificing the quality of course. Hehehe! How should this be done the right way? Peace out! Thanks in advance!