I finally broke down and picked up a pair of Marconi L63 grey glass. It has been a while since I have listened to my well used B65 but my gut feeling is that tonally the two are close to identical. The usual 6J5 advantages, such as improved separation and imaging, apply of course. I fully expect the prices on these to keep quickly climbing skyward as more and more people realize they can get the B65 sound through adapter usage.
To my ears they sound substantially different from the GEC L63 straight glass too. The presentation is quite a bit different IMO. The GEC presents with a much denser stage and dare I say is superior at pulling out low level texture detail. The Marconi grey glass is quite a bit more open and airy. Hard to really call one better than the other..... I think a better way to think of it would be in terms of what headphones you are matching them to. Paired with something that is already very open sounding like an HD800 I could see the Marconi leaning to the thin side, so the GEC would probably provide a nice counterbalance there for example.
The coke bottle Raytheons, especially the round plate version, are actually fairly close to the Marconi tonal presentation too. Not super close, but there is definitely a resemblance compared to some of the others that are clearly different flavors. Mostly the Marconi is just one step higher in overall refinement and finesse, but you certainly pay a premium for what is ultimately a fairly subtle improvement.
I'm glad I stopped hunting for a pristine pair that I would then not want to actually use in an amp. The ones I have test well but kind of look like mutts so I will have no qualms actually running them when I feel like it.