I recently installed 2 par of Polk Db651 and they sound great however I feel like I can get more out of them. Should I get an amp for my speakers or get a powered compact subwoofer? I am on the fence of which one. Any update will be appreciated.

 

A little tough to answer. If you like the output from the Polks, then get a powered sub to fill in the missing lows.
If you like what you hear, but just want a bit more, then amp the Polks.
It's all really up to you and there are more than these 2 options available.

 

id get a new HU (get a pioneer) unless u already have one.

 

I would upgrade the HU as well and add a sub. An upgraded HU will provide more power to the door speakers and the sub added bass (if you like that sorta thing). I am partial to Alpine and Harmon Kardon but any major brand is good. Pioneer, Sony, Kenwood, JBL, Rockford.