When the Helix XLR outputs are subjected to phantom power, the output level is decreased. Do not try and connect your Helix to a mixer, interface, or PA system that supplies phantom power (48v) to the Helix XLR outputs. The workaround is to use the 1/4" outputs instead. If your setup requires XLR usage and phantom power cannot be avoided, we've seen some customers have success with a phantom power suppressor.
When using the XLR input with a microphone, do not "hot swap" the cable. Do not have the 48v phantom power engaged when plugging in an XLR cable. It's best practice to plug in the cable with a mic attached to the other end, then go into Global Settings>Ins/Outs and turn on the Mic Phantom Power. Make sure to disengage the 48v before unplugging the cable as well.