Barring catastrophic failures like filament breakage or shorts, a decent 300B should last several years. By design, type 300B is a long life tube. Even the cheaper Russian or Chinese 300Bs should outlast most indirectly heated octal power tubes by at least a factor of two, at minimum.According to everything I've studied about DHTs, your EH 300Bs are probably just now hitting full cathode activation, where they have reached their full emissions potential. Different manufacturers dope their cathodes differently. Some apply an oxide doping formula that allows the tube to reach it's full emissions potential quickly, at the expense of some absolute lifespan. Others may formulate the cathode coating to give maximum lifespan, at the expense of reaching full potential more slowly.
Eric B. is the ultimate authority on stuff like this. He's mighty busy and isn't seen much around these parts, but perhaps he'll see this and chime in, or maybe someone else with a definitive answer.
Thermionic 
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