Scott,
 
The RCA tube manual calls for a 232 ohm (preferably 20 watt) common cathode resistance for a push-pull AB1 6CA7
(EL34) output stage, at 450 volts B+, 6500 ohms plate load. This seems to indicate that you should use the 8 ohm tap for a 16
ohm cab when using cathode bias, but whatever sounds best to you should be fine.
 
Ray
 
Mullard used 470ohm cathode resistors for el34s in their well known 4/20 amp design.With c.440v on the plates,round about
typical 50w amp voltages.
 
Of course this is the same sort of biasing range you will get with a c.250ohm shared resistor as suggested by Ray.
 
cheers phil.h