@charles1dad, the 6SN7 benefits from economy of scale. More designers use the tube because they are cheap and readily available, and demand for the tube motivates more manufacturers to offer the tube, which in turn drives the price of the tube down.
The 101D never benefited from any of this. It is tempting to draw a parallel between EL34 and 301B. 301B was always and always will be more expensive to manufacture than the EL34.
Israel Blume stuck his neck way out when he designed the CSL around the 101D. It wasn't a great preamp when all you could get was the Shuguang. Kudos to him for taking the risk.
It is a funny thing about tubes. You can go for years, trying this tube and that, and they all have their virtues and weaknesses, then all of a sudden, you find that one tube that is sonically perfect. For me, the Elrog 300B was that tube. The old Siemens from the 6922 family fell into that category. Got everything perfect, and nothing else came close to the Siemens. The WE101D replica is that tube. No where to go but down. Purity, transparency, detail, harmonics, tonality, it is all just right. Oh my, have we drifted from the OP.