Speaker Upgrade from JBL "Premium" System

I hope you are reading this. When you said " Low bass is not enough and highs are not crisp enough?"
It took my a few months to figure out why for the first time in my life I was turning off the music. Finally I turned the base almost all the way down and the sound went from "frankly disturbing" to only "simply deficient-in the low end". (I had put the bass all the way up to compensate for the absence of base.)
Since I do not want to spend really big bucks I am wondering between 2 options: 1) add a powered under seat sub and leave the headunit bass set to about 20% or 2) take out all 6 speakers and replace them all with better speakers that can handle the bass without going all muddy as they now do, which is why I am turning off the music, regardless of source. Any comments on either of these choices would be greatly appreciated. Clearly and new amp , and all new speakers and a sub etc would be best, but that is just not going to happen . Thanks to you and any others who can chime in on this.