I'm a new Atmasphere dealer but of the 3 x S30 amps and 2 x M60 amps I've sold I now only use a Russian 6H13C tube with the best sonic results and no arching. As Ralph states preconditioning really helps. On the side of the tube etched in a box on the glass it has the following:
BbInyCK
V1 - 823 r.
I bought over 200 in original NOS boxes but unfortunately I've sold all and my source in Russia is trying to procure more. If you can find these they are worth the asking price and in quantities are no more expensive than the 6AS7G tubes.
Good hunting
(Dealer disclaimer)