If it’s headphone-only, then put your smart phone mic up to your headphones and try recording that way.
So you’re saying this is definitely happening on the right side of the cue mix knob on your headphones? I have not tried the new firmware with headphone-only mixing, which is admittedly rare for me, but I’ve been able to get loud lately a lot.
You don’t have any speakers at all? How about a laptop? You can connect the X1800 to the laptop and listen to it through the USB on channels 9/10 on the laptop’s speakers.