I have a JBL EON Compact. It works great with my AT System 10 Pro wireless. I even power the receiver from the USB port on the speaker.
The mixer on the speaker has two modes; channel-mode and mix-mode. In mix mode, each knob is a volume control for a different channel. In channel mode, the top knob is the volume control, the next two are for EQ and the bottom one is for reverb. Maybe you are channel mode when you think you are in mix mode. This would explain the low gain. Sometimes it is easier to use the phone app to control the internal mixer. Things are much more obvious there.