I am 99% sure it is auto-bias based on a couple of recent conversations about tubes I have had with Craig. I don't exactly know what auto-bias means though so perhaps it is best just to ask him.
Also I take back what I said before about the driver tubes. After rolling the 5842's I think they will have an effect. It will be more subtle than changing the 2A3s, however it may be worth it for final tuning. The Amperex gold pins have been the final treble taming tweak with the HD800s that really sets the 2A3mk4 into another world compared with the BA I heard last weekend. It may be a slight roll of in the quality of bass, but I really can't be sure without doing some more testing. I would put the driver tube rolling slightly ahead of cables in the tweaking hierarchy. I will try to pick up all the 5842, 417A, 6C45 that I can to test out the effects.