You can adjust the bass frequency response of the JBL Flip 5 by switching its low frequency mode (LFM). So when ON, this makes the passive bass radiators move a lot more. Thus, LFM adds a hefty big-speaker quality to the content playing.
JBL Flip 5 Low Frequency Mode: How to Switch OFF and ON
Now by default, the Flip 5 has its low frequency mode turned ON. So in addition to the bass, this adds a bit of extra treble as well, to the sound. Thus, it’s like like switching on the Loudness control on older stereo systems.
You can turn LFM OFF by pressing and holding in the Volume Down (-) and Bluetooth buttons for about ten seconds. Further, hold them in until the oval that surrounds the Power and Bluetooth buttons lights up white. Find the Bluetooth button on the front panel island inside that oval. Furthermore, find the Volume Down button on the top of the Flip 5.
Then, to turn low frequency mode back ON, again, press the Volume Down and Bluetooth buttons. Hold them in until the Power and Bluetooth buttons light up again. Release the buttons when you see that.
Next, after a short pause, low frequency mode comes ON once again.