Even sub-1400 shouldn't need more than ~850mv, so it seems like you most likely have a problem in your ODT profiles or ppts:

1. Check that you're SOC is running 1107 and not 1200.  I assume it is, since you say your cards crash above 1107, but SOC is the primary reason i see jumps from sub-850 to above 875mv.

2. I've noticed in my testing that I can have cards crash if lower states aren't sufficiently powered - seems they can trip up during the brief transition across states during miner startup.  Solution is to either disable all lower states before starting the miner, or carry your top voltage all the way down to at least p2 (if you're using super low voltages like me.)

3. I have no definitive proof, but I have gotten the sense over many tests that the GPU threads aren't super isolated - i.e. one GPU struggling w/ insufficient power may cause another GPU to crash first, making it rather difficult to determine where your problem really lies.  Even if it's not necessarily the case, it's good practice to tune the GPUs one at a time - only adding another to the list once you've ensured stability of those already online (over hours, not minutes.)

Lastly, i should mention i'm on Sapphire Nitros, though i do have a single ref 64 i have yet to switch over, but which has always performed within the top third performance-wise of my nitros, so I don't think there's a inherent difference, other than cooling design.

my vegas 64 liquid didn't want to go below 875mv (850 in gpu-z) with soc clock 1107 (needed for mem clock 1100). It results in 19.6khs

• Windows 10 pro w/ 18.11.x drivers produced exact same results at my efficiency settings in linux, so didn't bother testing further. In general, I find w/ most algos that as long as the effective (under load) clocks match, h/rs and power use will match as well.  The only real diff between linux and windows is the cclock setting required (much higher) to get to your desired effective cclock.