I don't have Merlin's book but I guess it explains graphical loadline construction in PP amps. The primary impedance (Raa, Zaa) corresponds to 4 times the inverse slope of the optimal loadline.
A quick check shows that the 6.6k tap should be closest. Higher Raa will reduce output power and increase screen losses.
This old OSRAM datasheet gives some examples:
You want to look for the fixed bias configuration.
This said, original JTM45 with Drake OTs used 8K. The earlier RS OTs had 6.6K and 8k taps.