The EX-L sedan with Navi does not have the premium sound. The only sedan with premium sound is an Si. The Premium Audio System sounds good on paper, but in the Civic, it sounds awful. I had a loaner Civic sedan from the dealer and the 160-watt system sounded better than the Premium system, and had better lows without a subwoofer. I know that is hard to believe. The rear speakers in the Civic with Premium are crap. Small tiny magnets, about an inch in diameter. I am sure if you look in the trunk of your sedan, your rear speakers have larger magnets. The free-air sub in the rear deck of the coupe does absolutely nothing. I had to put my fingers on the woofer to feel if it was actually working, it was that weak. The front door speakers had much larger magnets and produced better lows than the rears with the subwoofer. It was disappointing because I had a Honda Element with the same premium sound, 360-watt amp and subwoofer and it sounded great with a decent thump for lows.
The problem with the Premium system in the Civic is the factory amp. It has horrible equalization. The factory amp has built-in crossovers for the front speakers/tweeters and the sub. It does not use separate crossovers. Replacing the speakers and keeping the factory amp does not help. So those who upgrade their Premium system, remove the factory amp and speakers. The factory amp does not make it sound better as you increase the volume. It actually sounds worse because it sounds more harsh in the mid-range with no adjustment available, because of the lousy equalization of the amp.
Now to answer your question. No, you cannot take an amp and sub from a coupe and install it in your sedan as a simple plug-and-play application. Your sedan has the wrong head unit, and is not wired for the external amp. The non-premium stereos are powered with an internal 160-watt amp and high-level speaker outputs. The Premium head units (non-navi and navi) are unpowered with pre-amp level outputs. You cannot wire your non-premium head unit to the factory amp without using a line-out converter first. The front speakers in the sedan must also be re-wired and replaced with the speakers and tweeters from the premium system. The sedan does not use real tweeters. The tweeters are wired off the front speakers with a resister cap on them. The Premium system uses real tweeters and the factory amp includes separate speaker outputs for the door speakers and the tweeters. So you would have to run new, separate wiring for the front tweeters, because the sedan's existing speaker wiring in the front won't work. You could probably use the existing door wiring since you would run new wiring to the tweeters. Could it be done with re-wiring and a line out converter, yes. Is it worth the trouble to add the factory amp and factory subwoofer, NO. You won't improve the sound by adding the factory amp and sub. You are much better off replacing the door speakers and rear deck speakers for better sound. You can leave the fake tweeters installed and they will still work. A full range signal goes to your doors and rear deck. Or you could add a line out converter, do an aftermarket amp and new speakers for better sound. That would be an improvement even without a sub.
By the way, I still have my factory speakers, factory sub, and factory amp removed from my Civic with Premium sound. The tweeters were destroyed when removed from the pillars because they were glued in by Honda. You can see pics of them here:
http://9thcivic.com/forum/threads/2013-civic-premium-audio-system-navigation-stereo-upgrade.10207/
