We have one in the books but not convinced it's a wise choice.
Ultimately we can modify an Agartha/Rockwell to run 2A3s. Largely possible for a red October to have done as well.
It's not a tube that there's been a ground swell toward when the 300b is an available option.
Id suggest that a 6B4G is a more interesting tube... its exactly the same as a 6A3 but with a 8pin pin out... It's easy to get 6G4Gs that are 60 years old on the cheap.
Id also suggest that a 1626 is very similar sonically to a 45.
What do you like about the 2A3.
From what I can tell, plus what others have said 2A3's tend to be more detailed and precise with less midrange bloom than 300B's have. As for bass, they're probably evenly matched, but I think the 2A3 is less laid back.
It'd be awesome to have a Red October v2 that could run both 300B's and 2A3's if that's somehow possible. Maybe even 45/1626's