Thanks Utopia for more info.
As a curious info I mention below, some other GM70 input stage tubes:
De Havilland: 12SN7 + 300B + GM70 old model
De Havilland: 12SN7 + 6AU5 + GM70 new model
SAC-Thailand: 6SL7 + 2A3 + GM70 SET from a custom order
SAC-Thailand: 6N1P + 6BL7 + interstage to two GM70 PP=70W
300B is a great drive tube, being a triode, DHT and plenty of power to drive the GM70.