Hi Robert (Rwanda),
I could not find schematics of the Cary 300 SEI...But the description says 15W(!!!) watt per chanel (out of ONE 300B!).
May be, they use class A2 conditions (driving the tubes to positive grid voltage) or they use the absolute maximum operating conditions (450V/80mA) to get this 15W...
If this amps have a bias control and You can regulate the bias to 70-80mA, ER300B could be used successful. But I can not suggest this at the moment.
Best regards, Matthias