Been in contact with Seasonic for the past 2 weeks concerning a faulty S12-600 Rev. A1.
Just heard back from them today that they have a Rev. 3 that DEFINITELY fixes the DFI boot problems (at least that is what I am told). Their RMA contact told me the Rev. 3 S12-500 and S12-600 PSUs will be available beginning Feb. 15th.
If you still have problems with your S12, I highly recommend that you contact Seasonic for an RMA.