At TSDL the russian 6N3P is listed as a possible sub for the 5670. These tubes go for $1 or $2 on evilbay, so they are cheap even if you have just 1 in 5 that works OK. I have been unable to find plate curves for it so I don't know if it would really work. Anyhow, I have ordered a few to experiment with.
Edited grammar.
Edited grammar.