The LR-16 has an input attenuator where you can set +8.2dBu or +20dBu input sensitivity.
As your request require hardware modification it cannot be solved by firmware update.
BTW. You can try to reduce the volume in the uTool mixer where you create the song. The default mixer level is 0dB in this case with full scale signal you’ll have 2 Vrms on the outputs. You can try to reduce the mixer levels and enable the post fader output in the menu, in this case you can drive your guitar amp from the LP-16 outputs.