Rolling Tubes in the Atma-Sphere C-30 amp

I just received this message from an Internet friend. I just emailed him for an explanation. For someone like me who is not technically advanced, can anyone comment in simple English so that I can understand what is going on here?

"DO NOT PUT THE RPs IN ANY LOCATION EXCEPT THE FRONT 2 TUBES !!! This is VERY important because the drive tubes are GTBs which have a rating of 450V - which is way beyond what the RPs can handle."

The use of GTB's in the last two rows was also mentioned to me by one of the Atma-Sphere resources. My guess is this may be due to the variants of 6SN7 (GT, GTA, GTB, WGT, etc.) tubes having different voltage specifications, but I am not certain.

Personally I have used NOS Sylvania GTB tubes in the last row, but went back to the stock tubes.

In the S-30, the rear tube has a little more voltage on it. If you are using NOS tubes, a -GTA or -GTB is recommended. A GT will also work but will not last as long- it is running right at its limits. So save your expensive KenRads or whatever for the front two positions.

Modern 6SN7s are marked 'GT', like the Chinese 6SN7; they work fine (that is what we use of course) in any driver tube location.