You can change the bass response of the JBL Charge 3, by turning on and off its extra bass mode. In fact, when activated, this mode makes the passive bass radiators vibrate a lot more. This also adds a rumble quality to the playing program. But you may wish to switch OFF bass mode when playing the speaker loudly, say at an outdoor party. This helps cut bass overload when playing very loudly.
How to Turn On JBL Charge 3 Bass Mode
First, by default, the JBL Charge 3 has its bass mode already ON after a reset. Bass mode also adds a bit of extra treble but lots of extra bass to the audio.

You can turn bass mode ON or OFF by pressing the Volume Down (-) and Bluetooth buttons.
Then hold them in until some of the battery gauge lights come on.
Find the buttons on the top of the speaker.

Finally, to reverse what you just did, press the Volume Down and Bluetooth buttons again. Hold them in until the battery gauge lights up again. Then release them.
Next, when the battery gauge lights come on, the bass mode switches back to what it was before. Especially for a power bank speaker like the Charge 3, you may not always want bass mode ON. Instead, you may wish to conserve battery power for recharging your phone. Or you may use the speaker outdoors, where bass doesn’t travel very far anyhow. In any case, having the most bass response is not always desirable. But do what’s best in your particular situation.