LibiaHong

I’d try forgetting the device in your Bluetooth settings and pairing it again, then check whether the volume issue happens with other devices too. Also, make sure the volume is turned up on both the phone and the Line 6 unit, since they may have separate volume controls. If it started after an update, checking the firmware version might help as well.

If you want to mute only the main output while keeping the other outputs active, check whether your device supports assigning a separate mute function or using an external mute switch on the main output path. The exact setup depends on which Line 6 unit you're using, so the model and current output routing would help narrow down the best solution.