Well I wasn't able to get 8x500 stable at 1.33750v (1.31v actual). I tried a lot of combinations of MCH/VTT to no prevail.

The most success I had was with the VTT at 1.28, MCH at 1.26, and MCH ref at 0.70v which was stable for 2 hours and 10min.
Changing the MCH ref didn't help and raising the MCH didn't help.
Bumping up the Vcore to 1.34375v (still 1.31v actual) seemed to do the trick at 1.28v VTT and 1.36v MCH with the MCH ref at 0.70v. I'm gonna wait until tonight to run a longer prime95 test on it.

It seems I don't have a golden egg here but I still feel it's safe to run this chip at 4GHz 24/7. Now it's time to push up the vcore and get this baby to 4.25GHz!