I just wanted to report that after recent evaluations I have finally found another driver/amp. tube for the WA6SE that significantly beats the 6FD7 (my favorite for quite a while up to now). This is the 6CS7, available inexpensively from Ebay. I am using a pair of 6CS7 to 6DE7 adapters made by Deyan. In detail, the comparison shows that at least on my system and with my ears, the 6CS7s significantly improve overall clarity and lucidity. A subtle fuzzy smear and overbrightness when using the 6FD7s is removed, revealing significantly more and more natural musical information, timbres, textures, transients. More highly resolved. I don't ever want to go back to the 6FD7.
I also have evaluated the 6SN7 (old NOS Raytheons) in this application, using another pair of adapters made by Deyan. I prefer the 6CS7s - they still sound a little smeared compared to the 6CS7s.
I should probably give some information on my system: The CD transport is an Ayon CD07s, the DAC is a Schiit Yggdrasil, and the headphones either HiFiMan HE1000V2 (preferred) or Stefan Audio Arts-modified Sennheiser HD800S. My evaluation so far has been mainly with classical music, and a little pop. I suppose it's possible that some sort of anomaly might show up with other types of music like heavy rock, jazz or full organ. I'm planning on trying these out soon.
At least it isn't an expensive experiment.