Can You Get Better Bass in a Car Without an Amp or Subwoofer?
That being the case, upgrading your stock car speakers can still yield some pretty powerful results.
The sound quality will almost certainly improve, but the bass will not boom.

Maica / E+ / Getty Images
For instance, it’s always possible that the tone control parameters were changed without your knowledge.
When an external amplifier has speaker-level inputs, you don’t have to worry as much about that distortion.
Can You Install a Powered Subwoofer Yourself?
Are Dedicated Subwoofer Amps Necessary for Good Bass?
The other options are to use a speaker-to-line-level converter or upgrade to a head unit that offers line-level outputs.
Turn the head unit on and set all of the tone controls to their middle options.
Play a piece of music you are familiar with that includes high, mid-range, and very low notes.
Adjust the volume on the head unit to between 25 and 75 percent of max.
Slowly turn up the gain on the amplifier until you hearclipping.
Back off the gain until the distortion goes away.
This basically means just swapping the position of the wires that connect the amp to the sub.
However, you will have to re-tune the system after making a switch like that.