upgrading 2014 JBL/NAV sub

So I've done a lot of reading and so far i'm not too excited by what I read. Is it possible to get a clean signal for a sub amp out of this radio somewhere? I read a lot of complaints about a grounding hiss and such when using a line converter.

The JBL system sounds pretty good overall but needs better sub response, unfortunately the rear sub seems tied into the over all bass setting which makes the door speakers sloppy...

 

The JBL system is pretty much as good as it will get. With low end components and a crappy amp there isn't much you can do to make it better. Most people just ditch the sub all together and go aftermarket for a sub.