2015 hybrid touring subwoofer
 

Im plannning to replace the infinity 8inch sub from the original.

2 questions

• for the connector, i want to use the Metra 72-7800 (on amazon). do you think i can use it for subwoofer? (probably same as 2014)
  
• i also want to know about rear speaker specs. its good, but if it can upgrade, then i should replace them at once. (probably change to KFC-1695PS)

the rear speakers are by Foxtex. you will downgrade them with cheap coax. the stock system attenuates the rear speakers on purpose - you shouldn't notice rear speakers from the front - and everything should come from the front.

a straight sub swap won't make much of a difference either. the stock amp is the limitation and those who have gone for more power in the rear deck just have a ton of rattles. with enough deadener and foam you can increase the sub output without inducing ton of rattles, but it's more time than money.