You can change how loud the low end sound is on this speaker, by turning on and off its bass boost mode. When switched on, this mode makes the passive bass radiators vibrate much more, and adds a low rumble quality to the playing program, that travels pretty far from the speaker. So, for you boom lovers, this post shows How to Bass Boost JBL Xtreme 2 Speaker speaker, to get that extra umph.
How to Bass Boost JBL Xtreme 2 Speaker: How to Turn It OFF
By default, this speaker has bass boost mode turned ON after a reset. This adds a bit of treble and lots of extra bass to the audio output as compared to when this mode is OFF.

You can turn this extra bass mode off by pressing and holding in the Volume Down (-) and Bluetooth buttons as pointed out by the green arrows in the last picture.
Hold for about ten seconds, until the Power and Connect buttons light up white. See these lamps circled in the next picture.

How to Bass Boost JBL Xtreme 2 Speaker: How to Turn It ON
To enable bass boost once more, press and hold the Volume Down and Bluetooth buttons in until the Power and Connect buttons light white. Then release the buttons. When the buttons inside the circled oval above light white, bass boost has resumed once again.
Warning
The drivers really thump when you switch ON bass boost. So be cautious in how loudly you play this speaker with the bass boosted, to avoid damage to the speaker drivers. If the bass is crackling, tapping, or otherwise distorting, turn it down !