Greetings,
Many have had your same complaint. You said latest BIOS, but a version number is more helpful, as latest today might not be latest tomorrow.
It has been said that F21 is the safest "best" BIOS for this board unless you are F8 or earlier. F20 was reported as a failure.
I would not recommend negatively under-volting your system to achieve a desired temp threshold as I believe you will only introduce instability elsewhere. Stepping and voltage are tied together in unexplainable ways on this series of boards. Stock voltage for 6700K 1.2v. I don't discount what you are seeing. I'm sure idle temps are 10-20c above what you were anticipating. Things about your system you left out.... Almost everything. Case, Cooling, brand / type RAM, etc. You can evaluate if your have adequate cooling. But again, many have complained of this issue with this series chipset.