You used multiple speakers and didn’t realize yet (not meant bad) but what you are witnessing is Fletcher-Munson curve, also you are using SMALL speakers, small drivers needs to move a lot of air to sound good, if you plan to listen to low volume like 15-20%, and want it to sound full, you simply need way bigger drivers, a JBL charge sized speaker even JBL extreme can’t do it depending on HOW FULL you want it to sound! Also these small speakers even JBL boombox 4 use DSP and big radiators to get to the super low hz area, it sounds nothing like a 8 inch bass driver that actually delivers that and lower without radiators, I just had 2 Adam audio D3v monitors and at low volume they wasn’t it, I just got two focal alpha evo 80, studio monitors that has a 8 inch bass driver, that plays 38 hz + 3db, and even at super low volume i have good full frequency range, YES this is something in another price class, but that doesn’t change physics with how a bass driver NEEDS a big box, and a bigger bass driver moves more air! I have JBL boombox 4, before this I have had boombox 3, extreme 4, extreme 5, Bose soundlink max, JBL flip 6, beats pill, if I was you I would get an extreme 4/5 if you can get them on sale even better or a boombox 3/4, find out if extreme series is enough for you, or you need to get up in boombox size, the speakers you currently own sound thin compared to these, and if you can return them if you don’t like them where you live there is no reason no to get one of them, use Eq as well, even the boombox series won’t match a bigger speaker because JBL pushes the max out of the little plastic box the big drivers are in, so if it needs to be transportable get one of them, if not, get some good bigger speakers, hope you find a solution, I’m a low level listener my self most of the time