Leo didn't have to contend with "matched" tubes, as all were built to be pretty close back in the day. I remember running down to the (7-11) between sets, and buying 1 6L6 to install in a Twin, or Bassman. Now days there isn't as much quality control, so we stick with matched sets, and bias as necessary. The bias balance was Fender's way of coping with the varying differences in tubes starting to be built at the time. We all thought is was a good deal, as more musician friendly, as was the heater "hum balance".