Sapphire, reference card (blower turbine fan).
use
AMD -Radeon Software Adrenalin - program to set frequency at
1388 MHz ,800 mv.
or you can use -OverdriveNTool- program .
I choose a simple way for lowering power and a little mem OC, using MSI Afterburner, I just drop power limit to -41% for XT and -24% for non XT card, memory to all at 885MHz and got perfect stability for 106Watts consumption for XT cards and 114Watts for non XT card.
If I choose to lower voltage and freq (as you indicate above), I got some crashes, especially when lower VGpu under 900mV, so this one is the best option for me,
double click on it for full size:
https://postimg.cc/D47JcXft