After many hours of trying different settings in GPU-Tweak II, MSI AB, and WattMan, I finally figured out how to undervolt below 1V. I saw screen shots of GPU-Z showing people running at ~900mV, so I knew it was possible. The default VDDCI (memory controller voltage) in WattMan is 1V. In order to get the GPU voltage below 1V, I had to reduce the memory controller voltage as well.
Someone may have already pointed this out in one of the threads hear, but I couldn't find it.
With WattMan set to 900mV for VDDC and VDDCI, I'm seeing 887mV in GPU-Z, and total system 120V AC power consumption is 146W with a cheap 80+ bronze PSU. My hashrate is 26.1-26.3Mh with an Asus Rx 470 Strix clocked at 1025/1875(1.5Ghz strap).
you can keep default VDDCI (memory controller voltage) and use negative Voltage Offset to reduce gpu voltage below VDDCI ))