It's been years since I tried the plastic base 6H8C, they were less refined vs most others if I remember correctly.
Like Rusty I do use the metal-base version which are very different vs the plastic base tube and are excellent. There are many variations though, with the hole-plate 1578-marked tubes being the best according to many, but I've also heard the black-plate version without holes are even better, this is what I use and I think they are better vs most other NOS 6SN7 types. I also use the 6SL7 version, the 6H9C metal-base black-plate tubes and they are also excellent. I'm not willing to spend the $200+/pair price for the holey-plates. You can usually find the black non-holey plates for under $100/pair.