The JBL premium sound system has a large woofer in the bottom of each door, a small mid-range speaker next to the woofer, & tweeters next to the front door windows in the corner, & two full-range speakers near the rear seats. The woofer (not sub-woofer) handles all the low end. You can test the woofer by turning the BASS to max and TREBLE to minimum (make sure balance and fader are centered) and put your ear next to each woofer.
The woofer is usually the 1st speaker to have problems after 10 years or so. The outer edge of the woofer starts to degrade. You will usually hear a rattling sound out of the woofer when this happens.
NOTE: If no sound at all out of the woofer (tweeters are OK), you might take your car into a car stereo shop. They can put a speaker on the wires connected to the speaker and make sure your problem is not the radio.
PROBLEM: You can still hear sound out of the woofers but it is distorted
SOLUTION 1: Go to a car audio shop and have them install new woofers. These may cost $30+ each but the dealer cost is over $200 for each woofer (crazy).
SOLUTION 2: More involved but very cost effective. If you or a friend is handy, you can go to www.speakerworks.com
and order a new set of speaker outer rings (on the site they are called speaker SURROUND) that will replace the damaged outer rings on the woofers (replace on both woofers). They really work great and your speakers should last for many more years.
Hope this helps.