Ralph Karsten (AtmaSphere) contributes to Audiogon threads all of the time - his Audiogon user name is "Atmasphere" - you should ask him.
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Erfranke, I believe the S-30, like other Atma-Sphere amplifiers, will use only the 6AS7G output tube. Ralph can give you recommendations on specific brands but my best experience has been to buy them directly from Ralph.
As to the 6SN7 input tubes, I am very fond of the Sylvania brand 6SN7GTA and GTB tubes manufactured up until the 1970s. They sound very good in my MA-2 amps.
As the owner of Atma-sphere M60 mkII.3's I find the the RCA 6AS7 available and not to pricey. They sound much better than the stock Russian tubes. In speaking with Ralph Karsten he indicated the amps probably had a somewhat greater power output with the American tubes over the stock tubes. He indicated as much as 20%.
I have also run RCA 6080 tubes in these amps with success. I was advised not to run 6080's in models earlier than the .3 version of the M60 since the earlier versions of the circuit tend to cook them.
Finally, I agree with Rushton as far as Sylvania 6sn7GTB in these amps.