low mu works in our favor, as it dictates a larger valued cathode resistor, which will allow for greater voltage swing from the output. For the octal fan, the best choice might be the 6BX7, as its low 1300 ohm plate resistance and high perveance make for a solid Cathode Follower.
The 6BQ5 (EL84) works amazingly well as either a pentode or a triode-connected pentode with a cathode resistor of 1k and power supply voltage of as few as 170 volts. 

Determining the Cathode Resistor Value
The cathode resistor sets the idle current through the tube. The greater the voltage across this resistor, the greater the potential voltage swing. The trade-off is voltage swing for idle current and a good Cathode Follower requires current. How much current should be drawn by the Cathode Follower? Of course, different setups will require different amounts, but a good starting point might be 10 mA for moderate runs of interconnect and more, say 20 mA, for longer runs or low input impedances from the devices being driven.
 
One experimental method for establishing the correct resistor value would be to wire a 5 to 10k potentiometer in series with a 500 ohm resistor and tie the potentiometer's center tab to one of its outside tabs. This arrangement will allow for dynamically varying the cathode resistor while in use. Once an optimal setting is found, a VOM can be used to determine the total effective cathode resistor value and a single resistor of the same value can be wired in place of the potentiometer and 500 ohm resistor.   n