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.
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.