Tube life expectancy: Stax driver unit SRM-006tII

Hi, does anyone have experience with the origin and life expectancy of the 6FQ7/6CG7 in the Stax earspeaker driver unit SRM-006tII? Since Stax are very strict about this, I haven't opened up the case, so I don't know what make they use. Since I use the earspeakers not regularly, I don't leave them powered on when not in use. Any rolling findings with the unit?
Thanks