I got my new 13 Jku this month, sport w/o premium sound.
The first thing I noticed going from my Cadillac factory Bose system to this was the piss poor audio.
I read alot of posts and decided I wanted to keep the factory HU.
First I swapped out all the speakers with some rockford prime 2 ways in the back and components in the front w/ dynomat. I figured after seeing the stock paper speakers this had to be a decent upgrade. But all said and done the difference wasnt very noticeable, not getting the volume I was looking for.
I then installed a 5 channel earthquake amp under the rear seat and used speaker level input adapters. Figured if the amp didnt get me where I needed I could drop in a sub.
At this point the highs and mids sounded great, volume alot louder, but I decided to drop in a single 10" kicker comp and now I'm happy.
Point being it would have been alot easier to replace the HU, but if not and your going for a speaker upgrade, expect to have to install an amp as well.