The driver tube position in the M-60 requires either a -GTA or -GTB style of 6SN7.
As you face the front of the amp:
In the MkIIIs and later this is the position that sits by itself in the front.
In the long chassis MkII.3 it is the rear 6SN7.
In the MkII.2 and earlier its the rear right position.
All the other tubes can be GTs. The new Sophias, TJ Music and Psvane tubes are getting very good comments from our customers; essentially saying that these tubes have the strengths of the best NOS tubes without the weaknesses!! It appears though that they may not be as reliable but that is conjecture at this point.
The best of the NOS tubes for the GT positions has been reported universally to be the Ken Rad (which was never available in a -GTA or -GTB). That is followed by the metal base chrome-dome Sylvania (a WW2 tube) then the RCA Red Base, which is the same as the CBS Zalytron.
We don't mind if people roll tubes, just remember that the one position needs to have the higher voltage variant and otherwise don't drive yourself completely bonkers trying to sort out the best combination- IMO you are better off spending the money on LPs or the like rather than tubes on the collector market...
