The Shuguang 845B graphite plate matched pairs will work and sound fine in the Nagra amps.
The best of the best is the United Electronics's, but they are
very rare and very pricey. I would call them invaluable because you can't just buy them with money.
I like the Shuguang 845 C witch features metal plates.
For the first stages I like a pair of NOS 6SN7s and a pair of NOS 6SU7 GTY, getting very hard to find.
Euro Oil caps make 845s cleaner and smoother.