Treat it like a McIntosh. The CF section will have lowest output Z with high gain/high gm valves. Triode-strapped pentodes from the video amplifier family are mu of ~40 along with 40 mA/V transconductance.Smaller cathode resistor gets less impact to output Z w/o breaking the original circuit performance.
Add to this, most of the amps I have build don't need much gain from the linestage and we are into 12B4's for the amplifier half....
cheers,
Douglas