Technical question about Cary 300se amps

I am replacing the 6sn7 driver tubes in my Cary 300se monoblocks, but first must determine if the voltage across the plates is low enough (300 V max) to use 6sn7GT's or if I need to use 6sn7GTA's or 6sn7GTB's, which can operate at up to 450 V. I know I can contact Cary and will do so tomorrow, but my impatience got the best of me to try to figure it out sooner.

Thanks in advance.