Yes, when you have a family of tubes that have different values for me but are otherwise relatively similar, the higher mu tubes will require less bias voltage to reach the same current as their lower mu counterparts. If you had seen that set of voltages you posted with a 6080 in the socket, that would generally indicate a very dead 6080, as the C2A is adjusting the bias very low to get the desired current out of the cathode follower section, but this is normal for a 5998.
