These lamps are of excellent quality and long with my Fender Hot Rod Deluxe combo.
Fender tube amps (at least those I know) original lamps are equipped from the sorter and I always stayed in GT for the power stage.
Also am I come to GT after testing a number that therefore 12ax-7 on the pre-amp stage.
GT offers a fairly wide range of 6L6 and "B" are those that were fitted behind my HRDL. Since I installed the "RD" (White Grade 7) and, more recently, "GE" (grade 1).
All give excellent results for my taste and have a specific character.
The "B" has a lot of gain drive and more drive mode with a grain a bit aggressive while remaining fully balanced with clean bass net and without burrs.
Today, my preference is for "GE" but "B" is nonetheless a safe bet.