Ah, cool!
I did exactly that with my JBL Charge 2+ this summer. First with the Boss Katana Go, and then with the Zoom AMS-24.
Ideal for park playing, when I want to get rid of people around me 
It has two disadvantages:
- Lowest frequency is not very low, so I need to cut off at about 75Hz with EQ. Otherwise the output gets distorted.
- There is a slight latency. Still workable though…
Nevertheless, I just ordered one of the last JBL Charge 4 in Germany. It goes lower than the 2+ (60Hz), has about the same size, has USB-c and is cheap.
Unfortunately, newer JBL models don’t have aux-in anymore.