You can adjust the bass output of the JBL Flip 5 speaker by turning on and off its bass mode. So when ON, bass nide makes the passive bass radiators move a lot more. Plus, you get an bit more treble as well. But over all, bass mode adds a notably hefty bass quality to the content playing.
How to Turn ON JBL Flip 5 Speaker Bass Mode: Intro
Firstly, by default, this Flip 5 has its bass mode turned ON after a clean reset.

How to Turn On JBL Flip 5 Speaker Bass Mode: Step by Step
You can turn bass mode ON by pressing the Volume Down (-) and Bluetooth buttons. Hold them in for around ten seconds.
Keep holding them in until the oval that surrounds the Power and Bluetooth buttons lights up white.
Find the Bluetooth button on the front panel inside the oval. Then locate the Volume Down button near the top.

Then, to switch bass mode back to what it was before, then again press the Volume Down and Bluetooth buttons. Hold them in until the Power and Bluetooth buttons light up. Finally release them when you see that.
Then after a short pause, JBL Flip 5 Speaker goes back to what it was before.