[Arran]

If the tester says it's good keep the tube, the low filament voltage reading is unrelated to the tube. There can only be two causes for low filament voltage across a tube heater: (1) you are measuring from the wrong reference points (2) the rest of the tubes and ballast are dropping too much voltage from the string. However if something were dropping too much voltage across the string then the heater voltages would be lower on all tubes not just the 6K8 tube.
Regards
Arran