You can adjust the bass of the JBL Flip 5 speaker by turning ON and OFF its extra bass mode (EBM). So when EBM is ON, extra bass mode makes the passive bass radiators vibrate more. Thus, it adds a hefty bass quality to the music playing.
JBL Flip 5 Speaker Extra Bass Mode: How to Switch OFF and ON
Now by default, the Flip 5 has EBM turned ON. So in addition to the bass, this adds a bit more treble as well, to the audio output. Thus, it’s like like switching on the Loudness control on older stereo systems.
JBL Flip 5 Extra Bass Mode: How to Turn OFF and then ON Again
Turn extra bass mode OFF by pressing the Volume Down (-) and Bluetooth buttons. Hold them in for about ten seconds until the oval lights up white. Find the Bluetooth button on the front panel island inside that oval. And the Volume Down button is on the top.
Then, to turn extra bass mode back on, again, press the Volume Down and Bluetooth buttons. Hold these in until the Power and Bluetooth buttons light up again. Release them when you see that.
Next, after a short pause, extra bass mode comes ON once again.